There could be folks holding leveraged Bear ETFs or similar after last week's downturn, who were waiting to see how the market moved Monday morning to decide whether to sell or hold. I could see those folks losing quite a bit of money due to the inability to sell off those types of positions after the market reversed course on Monday.

I suspect you're right though, that it's mostly sour grapes concerning the opposite case - inability to buy as the market rallied.