You may have done pressure transfers from keg to keg during filtering or after fining. During pressure transfers, it's a good idea to have a bit of head pressure in the receiving keg, especially if you're transferring beer that is already carbonated. Having an adjustable pressure relief valve in place will allow you to consistently control the head pressure and give you more consistent results, without losing carbonation in the process.

Note: The very end of the pressure relief valve can be adjusted by unscrewing the knurled adjustment knob. If too tight and not adjustable by hand, wrap a towel around the adjustment knob and loosen with pliers. Then finger tighten or loosen to desired pressure.