Built up gas forces its way out when the skin is cut.
Onlookers get showerd in whale guts :)
Try and ignore the terrible music btw.
(Should any of you find footage of this w/out the terrible soundtrack I´ll be happy to swap it in)
Here is the link to that other clip where they actually used dynamite: http://www.videosift.com/video/Dead-Whale-Exploded-on-Beach