What is the cause of 2nd gear popping back into neutral?

I own a 1997 Honda Civic Ex (coupe) SOHC VTEC that has been having this issue with the manual transmission where 1st gear chugs and 2nd gear pops out back into neutral causing the gear to grind pretty hard. I've come across many forums all giving different reasons to why this happens. I'd like to get a more definitive answer (if possible) from anyone who has had issues like/or similar to this, any mechanics out there willing to send some wisdom and tips my way is greatly appreciated.

Synchromesh is failing. Time for a recon gearbox.

Hi you need a new gearbox as this one is so worn it needs to be replaced.

Yes it's all bad news I'm afraid. Time for an exchange gearbox as yours is knackered. Seriously consider a replacement clutch too as if it is original it will be close to death. When the gearbox is off the clutch is easily replaced with very little extra labour. If you neglect the clutch and it goes later on the gearbox has to come off again with consequent expenditure.

2nd gear SYNCHRO is out.