Why Cooked Rice Spills Over in a Pressure Cooker?

Chatti S Rao writes:
Professor Srinivas,a physics professor, heard an argument from his home kitchen on a Sunday afternoon. His son Tammu, trying to save his sister from the embarassment of a messy situation with her pressure cooker has stated that "Look mother, it is not sister's fault. There is a problem with the pressure cooker."

When Professor Srinivas took interest the entire episode was explained to him:
It all started when Pranya noticed that a lot of cooked rice is on the bottom floor of the pressure cooker mixed up with water. Pranya tried to clean up things before mother notices. It was not easy because the cooked rice was around the support plate kept at the bottom. Cooked rice was also mixed with bottom water. Anyway mother noticed this and instead of scolding, patiently explained to his daughter how she could have avoided this mess-up. "I told you, Pranya, that you should never keep three cups of rice in that vessel. Its capacity is limited to two cups."

"I know mom, I kept only one and half cups. As I know that Sundays we have time to make rotis too."

"Then you must have made the cooker whistle too many times. Each whistle expelled a little rice from the rice vessel and it fell on the bottom of the cooker," explained mother very scientifically.

"No, mom, I switched off the gas stove immediately after one whistle.I let the cooker cool down. Opened the lid. It came out very easily.That means there was no pressure left inside the cooker.I pulled out the rice vessel with the correct tongs. Rice got cooked well. I thought everything was fine, but then I noticed a good amount of cooked rice on the bottom."

Tammu, listening to all this, tried to save sister and said "Look, mother, I think there is something wrong with this old pressure cooker itself. I noticed this happening earlier also. But you kept saying that you, perhaps kept too much of rice to be cooked."
"What could be wrong with the cooker? We recently bought a new weight because the old one was not lifting for a long time."

Now is turn of the professor of physics to explain the phenomenon. Everybody was satisfied with his explanation and it was decided by the family to go for a new pressure cooker.
What explanation did the professor give? What laws of physics are involved? Any calculations?