Yes I'd agree - good old CMOS latchup.

Put limiting resistors (as high a value as you can get away with) on the inputs and outputs, and use catching diodes to Vcc & Vdd.

It may just be on one pin, so you can probably identify this by simple elimination. Suspect anything remotely inductive or connected to long leads.

When you identify the cause try to both eliminate it at source, and also protect the micro - belt & braces.