Monoprice Select Mini v2 reattach boden tube to feeder head
I had a similar issue with my bowden feed system, those could be the reasons:


the pneumatic connector is not keeping the pressure on the pipe -> replace
the Teflon pipe is faded/worn -> try to cut the end of the pipe
the hotend/nozzle is clogged -> clean/replace
the pipe is excessively bent and gives a lot of resistance -> that usualy needs pipe/tube replacement.
the nozzle temp is to low -> increase printing temperature


In some cases I had to cut the tube above the pneumatic connector and push it down to get it out as there was no way to  pull it back from the collar.