What are your first thoughts when you see this, especially at the trot? Horse landing toe first on the hind legs.

Are you talking both back legs?

Yes, or if they land heel first on one hind and toe first on the other. Either way

Have you checked for abscess's.. 

I would start at the heels and work my way up.  Heels be the first thing for sure  And look at the hairline if the hairline is angled above the front knee you could posiibly have neg. Plantar. double check with xray.

Sometimes too high or too low of a heel will cause it.  Also if the horse warms out of it, try adding stretching exercises to your routine.  I do after a brief warm up if the horse has been stalled, then stretch or if they are on turn out I do before I get on and always after I finish working my horse.  Sort of like if I skip my stretching after a work out my hamstrings knot up and I end up walking tiptoe until they loosen up.lol

Abcess (easy enough to rule out by presence of digital pulse or positive hoof tester)

or hock injury. Stifle injury usually lands flat but has less push off.