When adding gas to the tank of my Nissan Sentra it takes almost ten minutes to to run in 10 gallons. I can't lock down the trigger even on the slowest notch else it backs up and splashes out so I have to hold it all that time.

What causes this to happen?

Something isn't right. Sounds like something is clogged up. Take it to a mechanic, unless under warranty. If it is clogged, I wouldn't risk whatever it is falling into your gas tank.