There's a thread on this here - bitcointalk.org/index.php?topic=5201697
It appears to be Binance doing stuff with USDT.
The fact that the fee is so low is showing good practice. If you're in a hurry it's only a tiny bump in fees to bypass it. If it were a spam fest they'd be much higher in an attempt to cock block you.
Interesting.
I noticed that the huge increase did not affect Bitcoin fee and I was a bit surprised. It made me wonder whether something big was fixed in the past that I missed.
I wonder what binance did exactly that could increase the transactions to such extent without affecting the fee.