I found the cause.
Someone got scammed on the NBT/BTC pair on CCEDK exchange. They thought they were buying NBT for 0.004493201 BTC each, but there was no sell wall from NuBot, so they paid 0.04493201 BTC instead, and they spent 1.89 BTC. This means they tried to buy $420 of NBT and received only $42 worth. Ouch!
Because of this trade (there was more than one, but this was the largest), CMC thought NBT were worth $10 each. That skewed all NBT trading pairs, market caps, and volumes by 10 times, which is why the NBT volume showed as $1.5M instead of $150 000. That's also why it showed NSR as being worth many times more than reality, because of NSR/NBT pairs with NBT valued at $10.