Once you put the wine in a carboy, there are two things that can create pressure in the carboy.
1. Insufficient degassing. Gas (CO2) is still coming out of solution. If you have an airlock on the carboy this is a "Good Thing" (tm). The CO2 will just bubble out.
2. Continued fermentation or malolactic fermentation. Not so good. Put a lock on it and wait.
3. (OK, this isn't a cause, but a result.) Changes in barometric pressure. Say you cork a fully degassed still wine during high barometric pressure. Then a low comes through. Now the pressure is higher (slightly) in the carboy than in the surrounding air.
All of these can push a stopper out of a carboy. Use an airlock if you can, or check the stopper as often as possible.