this year the top third of one of my hornbeams budded but failed to leaf out. Its now late June and the twigs and buds appear to have died.The hornbeam next to it also appears to be struggling -- it leafed out very late and still has a few branches where the buds haven't opened.

Before I replace one (or both) trees what would have caused this? Is it possible the same thing will happen again? Both trees looked to be thriving until this year.