This may seem a peculiar question but I was watching this video on youtube:
http://www.youtube.com/watch?v=aB-iIFV9bEM
and I can't work out why the people are falling over...

Can anyone explain?
It's referred to as "being slayed in the Spirit".
I've experienced much of it in my life, and you have to be very wary of it, and discern with sharpness, because many believe that they are being touched by our Lord Jesus Christ, when in fact it's a counterfeit spirit of deception and seduction of the "angel of light", a orchestrative by sometimes falseness.
Many who are seduced and deceived, and through their desire, sensitivity, and feeling they keep on going up for prayer and keep on falling down on the floor. At this point it then becomes an obsession and an addiction.
The next phase of this counterfeit they then begin to stagger around as if intoxicated, and is referred to a "being drunk in the Spirit".
Now I know as well as most Christians that our Lord Jesus Christ wouldn't want to be embarrassing His Children with them continually going up just to fall on the floor believing that they are being touched by God.
Now on the other side of the record, they are not counterfeits, and many have been touched by our Almighty God, and receive a healing.
But, I can assure you that many get hooked on this phenomena, and by doing so, the "angel of light" comes in and takes over, and it all becomes a show.
So, this is where you use your Spiritual Gifts of discerning of spirits, and at the same time test the spirits as well.
Many are false, and then some are for real, and are working with the Holy Spirit.
As I said I've experienced it many times, but don't allow yourself to be fooled, because we live in those
"perilous days" of seducing and deceiving spirits.
I've seen cases where a number of people keep on going up and falling over, and shaking and rolling, and stagerring around like a drunk.
And when asked if they believe that it's the Holy Spirit that is coming onto them all the time. And they where challenged to test the spirits, by praying a prayer that if it's not of God then they don't want it. Then guess what happened they never fell over again or rocked and rolled, or staggerred around.
But, they still occasionally where touched by the Holy Spirit and did get a healing.
Be aware, beware and never be unaware.