The most common cause of crank seals coming out of the case is trying to start the engine when the crank case has fuel in it.  If the petcock is left on or the petcock does not shut the fuel off 100% to the carb and the carb float valve is not in tip top shape, the crankcase will fill up with fuel in a short time. 

Using thread lock will help keep the seals in place under normal operation but will not hold the seals in place when the bottom end has experienced a hydraulic liquid lock or a severe crankcase backfire. 

Some engines have a plate or a set of tabs that prevent the seals from pushing out of the crankcase.  When these engines experience a hydraulic liquid lock in the crankcase, the reed petals or the reed cage dividers will often break spewing reed cage fragments through the engine.

Consider yourself lucky that the seal pushed out of the case rather than destroy the reed valve.  When the ignition cover has a good gasket and the stator wire grommet is in good shape. the air leak is minimal.  The engine will usually run fine without experiencing a lean condition but may suffer a small power loss due to the increase in crankcase volume.