What causes the rear sensors to go off when put in reverse and the sensor does not go off?

Your talking about the parking assist sensors?
When you shift into reverse the parking assist sensor are activated. These sensors work like sonar emitting waves and receiving the echoing waves returned. So anything blocking these four sensors (like mud, snow, ice) will cause the beeping noise to sound. The beeping will increase in frequency the closer you get to an object.
When the beeping no longer happens it is because the system had detected a failure somewhere in the system. Whether a sensor, speaker or wiring is faulty all will cause the system to stop functioning. A compatible scan tool can communicate with the parking assist computer and receive any diagnostic fault codes the system has detected.
I hope I answered your question

Thank you.

Your welcome. Thanks for the question