Fibbonacci

I built a base about 8 blocks up, and had a walkway to it about 17 blocks with a support in the middle. the zombies broke the support. I'm not sure why. They would stop on the bridge and attack the block connected to the support. this seems like very odd behavior, and I would like to understand why the zombies are doing this to help my base design in the future. I have watched some videos from Izprebuilt, and GunsNerdsandSteel but I haven't seen an explanation.