From these pictures (especially the bottom picture) it almost does appear that the MDF expanded and pushed the vinyl seams apart and about the only thing that comes to my mind that would make the MDF expand like that is moisture. I see that this is the bottom of the speaker, has it ever been setting on a wet carpet or floor Ichigo?
Of course it will most certainly be very difficult to have a better understanding without seeing the speakers first hand.
Soory about the speaker, hopefully the issue can be rectified for you somehow.
