Sorry to hear about your injury. Yes, I did the exact same thing as you... RGBP on a smith with butt off.
One thing I don't necessarily believe attributed to this was MMA. At the time of my injury, I hadn't been to class in months. I was working 70+hr weeks and simply didn't have the time to go. Also, with the style of BJJ I have, I never really got caught in wicked armbars, kimuras, or anything that would have tweaked my joints. Come to think of it, I've NEVER left MMA practice other than feeling exhausted from burning so many calories. With my build, guys would typically get me on chokes.
Things I think attributed to mine...
1- A previous "minor" injury 2 or more years prior to the pec tearing. I think this old injury was my pec tendon being slightly torn at the time due to the placement of the bruise I had after the minor injury. The minor injury took place while doing smith machine bench presses with a moderate/med grip, butt off.
2- At the time of both injuries, I may have been holding a little too much water weight, which I think gave my body a false strength. The extra strength I had was too much for the connective tissue to withstand. I was lean enough to ignore bloat at the time... but now I'm anal about holding water. I just won't "do" anything that will cause fluid retention.
3- The first injury and the complete tear of the pec, I remember cooling down a little bit because both gyms had a good air conditioning systems. I was going for all out sets, so I waited a good bit to get my energy back up. When I went under the bar, my body was too far cooled down to attempt such heavy lifts. I now train at a faster pace unless I'm at this old school gym where there is no air conditioning and the temperature is always around 80 degrees.
4- The explosion of power from the stretched position. I remember both times being at the maximum amount of stretch and trying to power the weight up. Dante hinted at armbars tearing biceps. Well... think about that. You're at a fully extended position trying to fight against it. Sounds like a preacher curl with too much weight. I don't think many have torn their biceps while the arm was at a 90 degree or 135 degree angle. Same goes for pec tears. Most people tear the pec from the bottom of the movement, not the top. I've started going slower (even more slow than usual) on my movements for chest, biceps, quads, and hamstrings due to prevent any possible tearing, along with trying to avoid a complete lockout on certain exercises that might put the muscle in jeopardy.
Those are my thoughts on how and why.
I wish you the best. If your tendon is torn, by all means try to get it fixed. If the muscle is torn with the tendon still attached, consider yourself lucky and unlucky and just rest until you're able to train again. This is a terrible injury, but it's not the end of the world. PM me if you have any questions. I'd be happy to help.