http://techdocs.shimano.com/media/techdocs/content/cycle/SI/Nexus/InternalHub/SG-3C41/SI-35U0C-001_ENG_v1_m56577569830655189.pdf

I had the Nexus 4 speed on a TREK CLYDE with forward facing drop outs on an aluminum frame it was loaned to my sons friend, an athletic 22 year old 350 pound 7 foot "BIG TEXAS". He never dislodged the back axle. I suspect improper assembly (missing or incorrect anti rotation washer)