After Gab was deplatformed years ago Parler had to know it was a liability to work with a major platform instead of developing their own. After Gab built back up they were in a position to receive many folks looking for a platform.
I think for Amazon under the current structure they are in little danger of law suits as section 230 has given protection for such platforms. However, politicians have put them in a difficult situation in which on the one side some complain about viewpoint discrimination, and on the other some complain about not enough removal of incitement, hate speech etc.
There has been talk from various politicians regarding section 230 reform, which could put all of them out of business. So what at times looks like coordination to deplatform may be more coordination to save the industry from section 230 reform.
The perception of parler as toxic, and pressure to act both from their own employees, and politicians likely made the decision necessary from their standpoint.
They probably also looked at the political shift and realized that the Trump administration which was more interested in viewpoint discrimination is out, and now they have to guard more against politicians who feel they are not doing enough to prevent illegal, or distasteful speech.
If they are viewed as doing so out of good faith actions on their own part they are less likely to incur a change of the legislative framework which has helped them tremendously.