I think the upgrade is an unlikely driver in itself. I haven't seen an upgrade have this significant an impact. I know I wouldn't have bid up SIMO by 20% just knowing that an analyst had put a $12 price target on it. Those guys are a dime a dozen, and they're wrong as often as they are right.
Earnings leak would probably be my top guess, but they'd have to be beating numbers by a sizable margin for the stock to jump as it did.
Potential acquisition rumor/leak would be my second guess.
And an incompetent buyer would be my third guess (highly unlikely for a buyer this size to be so unsophisticated).
I'm curious to see if there is follow-through on this move over the next few days. If it is an earnings leak or an M&A leak, what's the magnitude of the news. Let's wait and see.