although horses do try to lean on you and and learn to roll/fall down to prevent your from messing with them. or from girthing up and saddle fit.
it is important to rule out health issues.
My horse did this to me several months ago (and a couple times before that, although he has a bad knee)
it could be from knees hurting (arthritis) or knees not being able to close all the way (heels left too high during trimming will cause the stay apparatus to malfunction, the stay apparatus is what ables the horse to lock his knees while standing. also knee arthritis with accumulation of synovial fluid will do this.)
do you notice his knees shaking/buckling while standing? or when the farrier picks a hind or a front hoof up?
if not it could have been a one time thing, or some of the other things mentioned above.
*Member of the Quality Free-Choice Hay/Pasture Feeders Society* *In Favor of Turning horses out as long as Possible*