Not sure what you mean.
Do you mean that while your operating the gun a shell is ejected from the magazine, or you're ejecting an unfired shell from the CHAMBER?
Often, a symptom of worn, dirt impacted, loose, or defective shell stops is a condition where when you pump the action, two shells are fed instead of one, or a condition where one shell is properly fed, but when the action is closed a second shell is push out of the magazine.
The way a pump gun feeds is, on the back stroke of the pump, one shell is released from the magazine and pushed into the action to be fed.
When the pump is pushed forward, the next shell in the magazine is allowed to move back slightly and is caught by the stop so it will be fed next.
If the shell stops have a problem, the next shell in the magazine "skips" past the stop and is ejected out of the gun entirely.
Check the shell stops for impacted dirt, loose, wear, damage.
Here's a link to a Mossberg owner's manual that will help with disassembly and inspection of the feed system.
This is for the Mossberg 500 series but also works for the Maverick.
Mossberg calls the parts the "cartridge stop" and "cartridge interrupter".
Inspection is found on page 9 of the manual.
http://www.stevespages.com/pdf/mossberg_500.pdf