noone can prove causality about anything bitcoin related for the most part. but, look at the correlation. look at the time that asics started mass coming out and the price (obviously many factors involved). the theory is that miners spent way too much on mining gear that probably wont roi in btc  so them selling at a loss didn't make sense so hence driving prices up (miners refusing to sell below fiat/btc cost) - mining costs are essentially a floor at the moment. there are many theories of course. regardless, people are going against alt coins for the wrong reasons.

it's purely a "business decision" where if profit > cost or opportunity cost than you do something.
100mh (effectively 100gh in sha-2) mining is going for 10k. there's obviously higher margins there for the time being.