See, for example, every bubble ever. "We don't need to worry about buying that thing for more than it seems to be worth, because prices are going up so we can always resell it for even more than that later!"

If they actually believed the market they were trading in was efficient they wouldn't believe that prices would continue to go up. They would expect them to follow the value of capital invested at that level of risk. Further - as applicable to any bubble that doesn't represent overinvestment in the entire stockmarket over all industries - they wouldn't jump on a given stock or group of stocks more than any other. They would buy random stocks from the market, probably distributed as widely as possible.

No, belief in an efficient market can only be used as a scapegoat here, not as a credible cause.