It was an attack. What you've linked shows the number of transactions that went through (on a particular day). The attacker was creating a lot of transactions in the 1-10 satoshis per byte range. This filled up the mempool.
Is the Bitcoin network under attack guys?
It was 2 days ago. It seems to have stopped.
But it takes longer confirmations to come.
If you included the recommended fee which was 60 satoshis/byte (still is), it doesn't.