Second gear randomly pops out of gear when shifting from first

[1/4atatime]

So a few months ago I was pulling out of my driveway and I shifted from 1st to second at around 2.5k like always and when I went to put the transmission into 2nd it slid in smooth but when I let off the clutch it popped out of gear like I was in neutral but the shifter was still in the 2nd slot. Well I didn't have it happen for a long time and so I thought it might have just been a fluke. Well it happened to me again today this time pulling out from a stop sign and then it almost happened when taking off from a light but it stayed in gear even though if felt like it was trying to pop out that time. Now I wasn't revving it high, I have only shifted from second to first while rolling like 2 or 3 times (I stopped doing this a while ago when I read it wasn't good in our cars to downshift to first) and shifting at higher RPMs has never caused any issues though I don't do it that often and I don't drag race it. I only have about 16.5k on the odometer.