[2021] Horde night zombies attacked planted trees away from me

If they can compute a path to you, including the possibility of going through a number of blocks to do so, then they’ll try that. If not, they’ll attack blocks around them somewhat randomly, and apparently trees count as blocks for these purposes. If you want them to go a certain way, you want to lay the path out for them, tower defense style, and not close yourself off and leave them to do as they will.