My M&K MX200 sub blew it's caps a few weeks ago. Thankfully, one of my mates is an audio tech and builds recording studios. So he's having a look at the moment
Subs = large current = large banks of caps in parallel to deliver even larger current to drive the loudspeakers. Once the caps get old or go bad, the resistance increases and as it blasts out current it heats up (power, thus heat = current squared * resistance). Also if something else shorts it can short the caps and once again.. = high heat and pop!