One thing i noticed during my third viewing: Lukes former lightsaber ignites faster with a different sound since he threw it away. Check various scences: Reys training at the rock, Rey VS Luke, Kylo ignites the saber to kill Snoke and the last Praetorian Guard.
Maybe the one Porg accidently changed the lightsabers configuration with his feet right before Rey picks it up?
I noticed in TFA that it sounds very different to how it did in ANH. I especially noticed this when Finn first ignited it.