Bias Pitbull Rockers are NOTORIOUS for that. The sidewalls are so stiff that they push off the bead. Only in the last year or two have mine softened to the point that I don't burp the inner beads even when glued. The tires are damn near indestructible so I guess it's a necessary evil. :-\
Mine are the 16.5' wheel too so its even worse. I have a set of HMMWV double beadlocks to fix it though. I just need to build them. I was looking at the great lake 4x4 plate centers with gussets. They would let me choose my back spacing some. This thing is so wide I'm not sure I want to run the press in centers with 3.5 BS.