I'm gonna have a stab at this and say that it wasn't spam.

When you reach capacity at any club, and people keep turning up, the queue starts to grow pretty quickly. That's what's happening.

Whenever Bitcoin is 'Raging' - in all the papers, price going up, optimism flying high, the TXN/s simply increase, by a noticeable factor (2x?). And since we are 'almost' at capacity, it only takes a small push for the mempool to start growing.

Now that things have chilled out again, we're back to normal..

That's my take on it.