Yes, pressure can increase inside a microwave under certain circumstances. When you heat food in a microwave, the water molecules inside the food absorb the microwave radiation and start to vibrate rapidly, which generates heat. This heat causes the water molecules to convert into steam, creating vapor pressure.
If the container or covering of the food is not permeable to steam or does not allow the steam to escape, the pressure inside the microwave can build up. The trapped steam will increase the pressure within the container, and if it exceeds the container's structural integrity, it could potentially cause the container to rupture or explode.
To prevent pressure build-up, it's important to use microwave-safe containers and covers that allow steam to escape or to leave a vent for steam to exit. It's also crucial to follow the manufacturer's guidelines and recommendations for safe microwave use.