Well you know, from my experience, it is an uncommon occurance. However, I have had some fermenters do that very same thing before. Slow down a bit and then all of sudden whammm. I don't know for sure what causes that but....here are my suspicions. I think it's just certain strains of yeast more than anything.....nothing else.....just an excentric yeast. When all was said and done....the beer drank just fine and nothing was lost.
Also......a couple other things could be going on.......number one if your sanitation is poor is could be an infection....if it tastes good this is probably not the case, but always keep that in mind.
Also....it could just be a yeast metabolism anomoly. If temp was a little low at first and then there was a sudden spike in temp either from yeast metabolism raising the fermenter temp or just an ambient spike.....that could have pushed the krausen up all of a sudden on day 3 and bang.....blow off. Or there could have been a population of stressed yeast that regained activity all of a sudden.....I don't know what yeast you used....where you got it and the like, but.....sometimes yeast can do strange things that you have never seen them do before and its usually because you did something a little bit different that you disregarded as being irrelevant and it actually caused an unexpected response.......that's my two cents. Take it for what its worth.
I would just let things run their course and treat the batch as normal.......sample the next time your process allows and go from there.
Shaggy