my late MIL was making potato bags for years and years- way before the 'specialty' batting came to play.. Not once did any of hers burn up.. and I steam mine for 8-10 min.. I like big potatoes.. lol..
Did you use full power?? (Should never use full power- 80% max )
some microwaves are higher watts than others, so check to see what yours is and lower the power setting accordingly..
Was the bag wet?? (Mine is extremely damp, inside and out)
Im sorry you had an issue... Glad no one was hurt..