You just got an explanation from someone who opposes the changes, and who didn't give you exactly a sympathetic explanation.
There are several things going on. The one that I think is acceptable, even though I don't like it, is based on a concern that many of our cultural assumptions were based on males being the norm. A generic person was described as "he". Siblings of unknown gender were referred to as "brethren." Even God was affected. He is called "father" even though of course he really isn't either male or female. Some of this language isn't present in the original. Greek has a word for human that isn't male or female, which is often used in the NT, but was translated "man." There were even a few studies showing that some girls did have difficulty thinking of Biblical passages as fully including them. (Now that this trend has taken hold, the number might reasonably increase.)
So for several decades there has been an attempt to use gender-neutral language, or to alternate male images with female. New translations of the Bible, including the latest NIV (not just the TNIV, but the mainstream NIV) are using neutral language. This often involves moving language to plural or making it more abstract, which I don't' like. Terminology for the Trinity was part of this. Father and Son are masculine. Nobody much liked calling God Mother, Daughter and Holy Spirit, if only because Jesus was in fact male. So a number of people tried (and to some extent still use) creator, redeemer and sustainer. These are, of course, Biblical terms for God. The problem is that the Trinity is about personal relationships within God, so replacing the personal terms Father and Son with functions is questionable. However they are Biblical terms, and many people did not intend anything unorthodox by using them. You'll often hear other language like this, calling God the God of Sarah and <list of women> rather than the God of Abraham and <list of men>. They have also located female imagery in the Bible, and it's there. E.g. Jesus' passage about wanting to gather Jerusalem like a mother hen her chicks. Moderate alternation of language involving feminine images seems acceptable to me, although I'm personally not that happy about it. Speaking of "mother Jesus" is presumably a reference to the passage about Jesus as mother hen. Whether the person using it is orthodox I'm not going to speculate. The Episcopal church has leaders who are more liberal than is typical of the PCUSA, and I see no trend for the PCUSA to move further in that direction.
"sophia" is a Greek term for wisdom. It's used in the NT, although not in the way this approach uses it. It is used in the Greek of the OT for the personification of Wisdom as quasi-divine in Proverbs (which unfortunately for this movement is probably part of the background for the Word, i.e. the Son, more than the Holy Spirit). It was thought that while father is inherently male, it would make sense to balance this by regarding the Holy Spirit -- who is often (incorrectly) thought of in impersonal terms -- as female some balance could be added to the Trinity. Again, I'm not that pleased, but if done carefully this is probably orthodox. The problem is that it isn't always done carefully. But conservatives do sometimes misrepresent this as worshipping a separate false God called sophia. Personally I consider it a perfectly understandable experiment that didn't quite work out, but some people do still think it's a good idea.
There are however other things also going on. There are more radical moves to feminize Christianity. Some theologians claim that Christianity is inherently male-oriented. The whole idea of one God, of demanding justice, or dying for people, is taken to be based on male psychology. This approach rejects the idea of redemptive death and the resurrection, and of one God. It wants to use supposed female approaches involving mother nature and softer views of God. All of these are, of course, rejected by Scripture. There's no question that there are people within the PCUSA who take this approach, just as there are Catholics and members of other traditions. Most in the PCUSA have rejected it, and I don't think it's likely to make any progress. But it's easy to see the change towards gender-free language and alternation of male and female-orieted imagery as representing this heretical tendency if you're hostile to all change.
I continue to say that all evidence shows the PCUSA as fairly stable in theology since 1925. The female language thing is more recent, because it was clearly sparked by a movement in the culture that's more recent, but the underlying theological approach has been the same for 85 years. If anything the last couple of decades the average Presbyterian pastor has probably gotten slightly more conservative. However this move is clearly making progress in the evangelical community, as represented by the change in the NIV. This bolsters my thesis that a good chunk of the evangelical movement is slowly moving to join the moderate mainline in theology, i.e. the PCUSA, not the left wing of the Episcopal Church.