My uninformed, unresearched 'gut feel' is that:
(1) 85mm AP could still have a relatively big bursting charge, and
(2) It might not be the most efficient fragment-generator when it explodes, but would at least cause relatively few, relatively large pieces to fly much further than an HE shell.
So maybe its fragments can hit troops fairly far away, although it is unlikely to do so.
Why it is set off immediately by a tree, and not from bouncing off armour plate, I have no idea.
FWIW, according to some googling, the 85mm AP in question may be BR-356, a 9.2kg AP tracer wi. flat nose & ballistic cap shell, containing 0.164 kg of "phlegmatized hexogen with powdered aluminum".