Just when I thought I had you all fooled, Judy slipped in at the last minute with the correct answer – the “pruners” in question are deer:
The pruning ends when the deer can’t stretch any further, giving the tree its odd bell-shaped crown:
This is also a great demonstration of how pruning stimulates new growth – you can see the dense healthy growth at the base of the “bell,” even though this part of the crown normally would be much sparser.
Thanks for all the guesses!