For anyone like me who made themselves feel bad by assuming this was some horrible symptom of climate change or pollution being ironically shared by the "nature is amazing" account and reposted here for laughs, I looked it up, and it seems to be a natural phenomenon! This happened in 2024 and the most likely theory is there was an upswell deep in the ocean, causing a plankton bloom, leading the fish to follow the plankton to shallow waters, leading to this. That or they somehow knew about the massive earthquake that hit the Philippines a few days after this. But this isn't the only time this has happened there.
Either way, probably not human-caused, yay!