Current speculation is that it's a novel object type, unlike the conglomerate iceballs we call "comets," with a different origin and history.
Unfortunately we did not make the heroic effort it would have taken to get a close look, so as with 1I/ʻOUMUAMUA we'll be left with enigmas and uncertainty.
Unless of course it brakes hard into a parking orbit. :)
Maybe it was cooking away like a sealed food container in a microwave, internal pressure building, then the seal is breached and the internal pressure is released in a single event.
