It has alot to do with his running style. I never fumbled once as a receiver, but i never ran very hard either (I always tried to get as far as i could after the catch, but since i was kind of a slow slot receiver i usually got hit directly after the catch, and when i got tackled i always made sure that my body was in a way that the ball was away from the tackler instead of trying to run people over). Im not very strong either (haha I was one of the least beastly guys on the entire field ;D) so the point i´m trying to make is that holding onto the ball is not that dependent on strength and how hard you hold onto the ball, it has alot to do with how much you expose the ball. If I had to take 20 hand offs/ game and go up the middle and get hit straight on by linebackers and stuff, i would have fumbled for sure.
When i ran after the catch, if i had the ball in the left arm, i made sure to absorb the tackle with my right shoulder, wrestle my way down to the ground if i was held up so that no one would even have the chance to strip (i actually invented a term for this "active ball defence" ;D, and since im kinda small and easy to bring down, i usually flew to the ground on the first hit before there was any opportunity for a strip. I know this isn't exactly textbook "tough football behaviour" (YOU GOTTA FIGHT FOR EVERY INCH; RUN PEOPLE OVER, BRING THE PAIN AND KEEP YOUR LEGS MOVING yadayadayada
) but i did achieve my goal to never fumble.
Beast mode on the other hand is the complete opposite, running EXTREMELY HARD, straight through people. It is imposible to employ his running style and still use active ball defence (going to the ground in a pile is the total anti beast mode). I do get frustrated that some of his fumbles just seem to be lack of concentration, but we get so much in return for his aggressiveness that the ocassional fumbles are worth it.