Well that was interesting. I called the dealer and spoke to the service dept. They were stumped as well. I got to thinking that with the vent tube it shouldnít be pressurized enough to push the other side out. I knocked the side back in that was now popped out, and it went the other way again. Iím thinking those 2 shafts are touching inside or only 1/6Ē apart. There must be a little slack where it can go past where it is at rest and one contacts the other shoving it out. There is a fine line between not enough hammer and to much hammer. If youíve never done a rear axle boot before on this machine and you donít need to tear down both sides be careful on the amount of oomph you give it. Good news for me was it led me to discover bad seals in the other knuckle before destroying wheel bearings!