i blame the difficulty algo.. it is rather interesting, if it ceases to be profitable, people leave, diff adjusts and it is profitable again.
Its been interesting watching the diff drop down to below 1000 and it still be profitable to mine. If you can sell that is. of course with a low diff, that means low interest, less miners, and lower volume at the exchanges, but the coins still appear to move, so go for it I say
The only problem with difficulty falling all the time is that soon little red riding hood with a pocket calculator can pull off a double spending attack.