I've seen a theory that companies are trying to get IPO'd now, in anticipation of a weaker market in the future.
I think this theory tends to be based on the idea that the current state of the stock market does not reflect reality for many businesses (and the economy as a whole), nor their future prospects, and that a correction is imminent.
Obviously this is speculation, but it's interesting because someone probably does know - there's probably some overlap in the advisory banks for these IPOs, and they must have some idea of the thinking of the executive teams of these companies.