On a widebody this can be a major problem. The slide itself, which is massive, will inflate with such pressure so quickly that anyone standing around could be swept up or trapped against the walls and be seriously injured. Additionally, modern slides are fairly puncture-proof, so it would be difficult to stab a hole in the slide to let the air out.
On the MD
-90 this is less of an issue, but with the limited space in the cabin it is still a dangerous situation.
The obvious answer is that the bottles holding the compressed air inadvertently fired and blew the slide. I'd be interested to see exactly why this happened, or if it had been tampered with.