Smith apparently gained a large amount of weight over the years since he was famous as a football player and actor. I think this pic
is from 1997 or so. This leads me to suspect a heart attack or stroke as the most likely cause, though it seems odd that the cause wasn't released. Usually that either means that the person died as the result of what is often considered some sort of moral failing (drugs, alcohol), or because they didn't want their fans to know they'd suffered (cancer, emphysema, etc.). So we really don't know and I guess we never will, but if I was going to bet, I'd bet on a heart attack or stroke.