I don’t know the exact thing; but I think it was discussed a bit in the news mega—basically there’s potential for corrosion and bad maintenance can lead to the stuff (whatever the latent state they’re keeping it in the tank) reacting and the reaction is thermal and sorta keeps itself going/getting worse—it puts off very toxic vapor (hence the evacuation) and they’re trying to slow or limit the reaction by cooling the system—not clear if that can actually avoid awful outcomes or just limit them by limiting the thermal potential overall, etc.?

Edit: here’s the link to the news mega response that has a lot better info for those who are curious https://vger.to/hexbear.net/comment/7188249