Why is the rear wheel of my tandem bike skidding? We have recently bought a tandem and the rear wheel skids out when riding. Any idea why this happens?

Comment below: When we are riding along in a straight line and there is an uneven surface it does this a lot. The heavier rider is on the front of the bike.
The captain's weight hardly pushes the rear wheel down at all, while the stoker's weight is almost completely on the rear wheel, so check their weight is positioned correctly. If the stoker is standing up, this can shift weight off the rear wheel, making a skid more likely. Standing up on tandems is "advanced", some would add "insanity".