Heroin Why would a persons jaw lock during an opiate overdose? I couldnt give cpr!

I've been struggling with heroin addiction for about 10 years so I been around the block a few times and witnessed a few overdoses in my time but this was SCARY. The other night my bf of 3 yrs od'ed in my presence for the first time, the bluish-grey, not breathing, dead od. He had shot 1/10 of heroin, then 1 hour later he shot another 1/10 that's when he fell out. He had also shot meth earlier that day, smoked it throughout, and a half a xanax a little earlier in the day as well. I heard a crash so I ran in pulled him in the hallway but when I started to administer cpr his jaw was locked tight, i could not get it open, so I tried hopelessly anyway being frantic, and rubbed his sternum, he was slightly responding while I was on the phone with paramedics. He's fine now so no worries, but I was hoping I could find out why his jaw was locked, preventing cpr and what to do in case this ever happens again. If anybody can answer this for me id greatly appreciate the peace of mind it will bring. Thank you