The zhá reading exists for one purpose only: the second syllable of
挣扎 (to struggle, physically or metaphorically). You never see
扎 standalone, and no other compound uses this reading. If you see the bigram
挣扎 — read zhēngzhá. If
扎 stands alone or pairs with anything else, it's almost certainly zhā (default verb) or zā (tie / bind).