Join Date: Sep 2009
Location: Cincinnati, OH (USA)
Doesn't look like a conformation issue -- she has decent angles to her croup, hocks & pasterns, and her hips look fairly even. To diagnose this problem, we would need to see pictures of her standing square, in profile and from behind. And ideally also video from the side and from behind of someone walking & trotting her in a straight line. It's impossible to tell anything from videos of her trotting a million miles away and meandering around in a field.
However, two things come to mind when I hear tripping issues with the hind feet. First, as you've already mentioned, EPM. The other, which would be far preferable, trimming/shoeing issues in the feet. If the medial-lateral balance is off in the hind feet, it would be pretty easy for her to interfere with herself. It almost looks like there's something going on in the walk, but it's hard to tell without seeing her move in a straight line.