Maybe in such a small tank if that is indeed a pico. I have many times inadvertently increased the salinity and also the ph to over 9 and never lost a zoanthi. I would agree that thee may have been something else, though pink mat polyps (candy apple pinks or whatever common name you wish to give) are a bit more tempermental than most other zoanthids.
I would also look toward the possibility that picos are a very sentitive system and any other number of possibilities such as killing benificial bacteria during the spike thereby causing excess ammonia leading to nitrates could have lead to their stress and demise.