This is how we pour the grain for my senior project. We're using hydroxyl-terminated polybutadiene (HTPB) with carbon black for energy retention. When we pour the rubber, we have to keep the bubbles out of it as much as possible, which means going slowly, but we also have to pour it within a certain amount of time after mixing in the last ingredient. This particular day, we actually didn't pour it fast enough and the fuel cured too much before we could actually finish pouring the grain.
Bubbles in rocket fuel cause pressure pockets. When you hit a pressure pocket during combustion, it can cause anomalies in the flow and result in catastrophic failure of your engine or motor. This is why we try to vacuum pump it and go slowly and keep it as bubble free as possible.
We also have grain liners to make sure that the grain doesn't fracture and blow out the back. Because that happened. And it sucks.