Why does my BB Classic randomly switch from headphones to speaker?

I have a BB Classic from work. A little while ago, at around 12 months old, it died on me and I was sent a brand new replacement. However, I have the same problem with the first phone and current phone.
I listen to podcasts on the train through headphones. Quite often the volume suddenly drops and I have to use the volume keys to put the volume back up. More rarely, the volume suddenly turns up by itself.
The worst thing though is that quite often it randomly stops playing the podcast and when I press play the volume will only come through the speaker.
The only way to fix this is to reboot, which is annoying and takes time.

It is not the headphones, and this happened to both new BB Classics.
It works perfectly most of the time, but just randomly and often does this.

If there was a setting to switch back to headphones I could live with it.

Could this be due to a sound profile setting? That is to say that a sound event is triggering the change when receiving an email/text/an app notification/phone ringing/etc.?

The connection between the headphones and the device may not be fully "sealed". By this I mean the headset may not be fully connected or it may have some wiggle room for whatever the case may be (defective units, not fully connected, etc.)