"I don't think he has a strong ace or a huge hand, if he did he would have bet the flop with such a flushy board. I don't have a big hand either, so I'll check the turn too.....ohh, he bets now, huh? Well he obviously doesn't have a strong ace, maybe I'll do a check-raise and win it right now. He calls? Whoops. If I want to win this pot I better fire out on the river! Whew, he folded. He probably had crap anyway."

Or something like that.

Maybe if you're seeing this move quite a bit more, your question shouldn't be "WTF does this mean?" Your question should be "why does this weird play keep happening to me," and *then* WTF does this mean?