For me it seems that there was uneven distribution of the electric charge during the running. I don't know which type of chamber you were using for the run.
If the vertical one most likely there was a leakage from one side of your gel therefore the charge was uneven. You will commonly see this wave when you have this problem.
If it was horizontal one, you might want to check if your wire on the chamber still intact so that it produces even charge.
Yes, I agree with Sutandy. This is the common problem with old electrophoretic units, arises when electrode knobs are not tight. But, you should also prepare fresh buffer and especially check the pH of water used for buffer preparation.