Official comment
This is because your headphones have a microphone. The other devices you are using your headphones with likely have a combination headphone out/mic in port, such as a smart phone.
Regular headphones have a plug that looks like this:

Headphones with mics have a plug that looks like this:

Notice the difference is that one plug has 2 black rings and the other has 3. You will need to buy an adapter, such as this one, in order to use a headphone/mic combo with your DDJ-RB.