For footprint chart users, what is going on here? Apart from normal volume traded at the bid and ask, I see this pattern occasionally, especially in NQ futures; abnormally high volume at the inside bid and ask of roughly the same size. Are these spread trades being executed or what?

Usually just means the market is "spinning" there for a short period.

Lets say market moves up and a large offer iceberg is there, as traders buy into it - it'll build volume.

But that doesn't mean it's one sided, the other side will still be trading, so you'll see size on both sides.

1. The answer is in the charts.
2. These charts are misleading.

What's missing here is the sequencing because I think these are volume charts right? In general when you get a lot of two sided trade it's because both buyers and sellers want to transact and there is liquidity to do so. It looks like in several places this is around the middle of the bar. On the other trades, the sequencing may not be showing what really happened unless you are using a resetting chart. Most of the footprint charts tabulate over time. So, you will notice in other places these occur at the edges of the range and that's probably because of the sequencing where stops were run and then buyers/sellers got aggressive and the market reversed.