How did little red worms get into my hot tub?

I’m pretty nature savvy but this baffles me. I drained my hot tub the day before yesterday and when I went to clean and refill it this afternoon there were two small red earthworm like worms swimming around in the few gallons of old water below the drain line. They were completely submerged but wriggling around alive.
How did worms manage to scale the exterior and get under the cover? Do earthworms climb and migrate towards water?
Bizarre!