What would cause the back wheel to lock up when in reverse?

A back wheel may lock up when in reverse due to several reasons, including a malfunctioning brake system, such as a stuck caliper or faulty brake pads, which can cause excessive friction. Additionally, issues with the transmission, such as improper gear engagement, can lead to wheel locking. Low tire pressure or uneven tire wear could also contribute to this issue, affecting traction and control. Lastly, an obstruction or mechanical failure in the drivetrain may also result in the wheel locking up.