My guess it was a pre-arranged trade.  Who puts up 280,000 shares on the ask, unless they expect to get filled at that price pretty quickly?  In "normal" trading, that would put incredible downward pressure, defeating the seller's purpose of getting filled at the highest possible price.

And the fact that it was put up on the Alpha is also telling.  Putting it up on the main board would scare most retail buyers who don't see an integrated feed.

The ask was taken out within 30 minutes of it being put up.