Hi, i have a problem with spark plug wires

Ok first of all i have a 97 Accord LX (non-vtec)

I was driving on the freeway when i noticed my car was slacking and losing acceleration power.

So i pull over and pop the hood.

I found that one out of the four spark plug wire was loose, it was not connected.

This had never happen to me before.

So i took a look at the wire and i saw oil.

Well..i did put the wire back in place and the car went back to normal.

Well..after a bit of driving..the wirer loosen again...and if anyone know the problem to this..i would be grateful..thank you.