I don't think a 100K sell order on QQQ is that unusual. I use to get block trade information on blocks over 250k and would see sometimes a dozen a day...most at 400-500K. I use to post a good amount of information here about them when they came through back about 2002. Schwab quit the service. Here's a post by someone else (can't find any of mine) from 2003.
Maybe they got a margin call in another position and need the money? We'll see a lot of these hedge positions do strange things as so many are married to short or long positions elsewhere. QQQ is a hedge vehicle for many, what happens with trades in the QQQ are often the result of other positions effects.