If you're curious what causes that stuff threatening Garden Grove, Orange Co., CA to heat up while sitting in a tank, this EU data sheet details how things can go wrong and what can be done.
To summarize - it’s complicated.
It’s unstable material with a shelf-life that needs babying all along, and it's tricky to arrest a runaway reaction once it gets going.
Finding root cause of this failure will be a whole other game.
https://www.petrochemistry.eu/wp-content/uploads/2018/01/Methacrylic-Esters-brochure-5-08.pdf