guitar string just randomly snapped while it was sitting?

I was just sitting down in my bed and all of a sudden i hear a loud twang noise from my guitar and my 5th string just snapped. I was playing it like five minutes beforehand in full step down tuning so there is no way it snapped from being too tight. I mean the strings could have been pretty cheap considering I got it in one of those all in one guitar kits (guitar, amp, and tuner) but the kit itself wasn’t cheap and it’s actually a pretty decent guitar (epiphone les paul jr). Is there any reason why this happened or can I just put the blame on cheap strings?