As I mentioned before, mixing low voltage with higher voltage signals is BAD. If you want a reliable system, implement the relays as I mentioned - your problems will magically disappear. Not only will it give you isolation, if you get relays with the inbuilt led, you'll have extra diagnostic capability. However, it sounds like you want to mod your existing circuit -try Kai's suggestions. Since the 74lcx245's have input protection (this is from memory -best be checking) the bidirectional transzorb is letting enough negative voltage through which the 74lcx245 input is catching. The energy must be enough to zap the input. As Kai suggests, a unidirectional transzorb will help here.

In bad situations like this you have two choices -
avoid or mitigate. You've decided to mitigate. I'd suggest you try avoidance first if you want reliability.