Ender 3 Bowden tube popping off
If the fitting is remaining attached to the PTFE tubing, that would indicate that the threaded end of the fitting is pulling out of the drive assembly. This implies that the internal threads of the drive assembly have stripped out. This is not unusual for a plastic drive assembly.
The best solution is to replace the drive assembly. I believe I paid about US$12 for the last one I purchased and it was aluminum, not plastic. A quick search for &quot;Ender 3 drive mechanism&quot; returned a number of choices. One of them from Amazon (14.98) is anodized aluminum and purports to be improved over the original.

A less than ideal solution would involve drilling out the stripped threads and installing an insert (sometimes called a Heli-coil™) but that could be as expensive as a replacement mechanism.