Why is my one goldfish trying to get stuck?

He's not trying to get stuck.

He was injured and doesn't have the strength to fight the current of the filter intake.

I don't believe goldfish "try" to do anything.

they try to die.

and they eat.

(and if you feed them too much they will eat till they die)

they are incredibly not very smart fish...

You should get a Beta...they live and live and live and live......and live.

He was trying to plot an elaberate escape plan. Ever see that on Finding Nemo?

Fish aren't too bright, and sometimes they aren't trying to do anything. At one pet store I frequented, there were some fish who'd all get stuck face first in a series of pipes that were in their tank. Every night the pet shop owner had to pluck them all out so they'd not die overnight. In this case they weren't trying to do anything other than investigate the hole they'd already forgotten about and they'd get stuck.

However what you describe sounds like it may actually be injured and trying to hide. Weak fish (sick, old, injured) will avoid open water. Do you have other places for the fish to hide? Can you block the filter with plants or ornaments?