n6394p
"why was most of the volume on puts and not calls?"
We will never know!
Your first thought may have been correct, they were bought in expectation of a fall in the QQQ, but you don't know if the high volume was part of another instruments high volume. By that I mean you are not seeing the whole picture.
You can sometimes see VERY HIGH volume in the S&P500 Puts (SPX) and also the Futures, this is probably buying Puts to protect the bought Futures.
If you are bullish on QQQ then it can be better to sell Puts rather than buy Calls, because if you are wrong you can still profit from the time decay working in your advantage.
USA Option exchanges have to trade a certain quantity of options at the curent Bid or Ask, so for every buyer there has to be a seller. Of course the person on the other side of your trade could be a Market Maker (who will hedge away his risk).
Keep looking for the Volume Spikes 🙂