I suspect, but don't know, that there was a flaw in the pan and there was a small hole or gap at the site of the catastrophe. The microwave induced electrical currents in the pan which shorted across that flaw, generating a lot of heat the exacerbated the damage and it cascaded from there.

In a flawless Pan, the electrical currents would harmlessly zip around without issue.

Frozen single serve meals with packaging like that are seriously risky.