I've had this in the past and it was a change of category that did it. Amazon sometimes does this for no apparent reason and it can be catastrophic for indexing and ranking. I would do the following:
- check each listing one at a time and check which sub category they are in
- check your BSR graph for each product. Has it suddenly shot up to a much higher number (reflecting a drop in rank)
- index check for all your main keywords. Are you still indexed
- use a keyword tracker to see if you have lost rank on your main keywords
- check the above for your competitors. Is it happening to everyone or just you?
Edit - I've just checked that first one and realise now that you are just one of several offers on the ASIN. Someone else has the buy box at the moment at the same price but price isn't the only factor. For example, you have a 92% seller rating whilst the current BB owner has 98%. That could be a factor. Also things like how much inventory do you have in the system, how well is it distributed and how well do you manage your inventory can affect the BB.