My gelding is about 3.5 years old. He's very good about picking up his feet, and was for the farrier, until I moved barns. He did not like the farrier at the new barn at all!
One of the things he started to do was to fidget when a front was picked up. He'd hold it up willingly, but after a few minutes, he'd start creeping his back feet underneath himself, then start sinking. Since this was combined with other behaviors, I thought it was an avoidance behavior. He did it with me for a few days after that first trim, then stopped for a while.
I no longer use that farrier, but he's persisted in this behavior. I was pulling a wrap off his foot in the field last night, and ignored the sinking, until he finally dropped to his knee!
Again, he'd be fine for a minute or two, then start shuffling his hinds, then start sinking. He's doing this pretty consistently now.
He doesn't appear to be lame, or have any particular heat. Maybe - maybe - just a very slight warmth on the inside of his knees (and hocks).
So my question is - have you seen this as an avoidance behavior, or do you think there might be a problem somewhere?