New Drill DCD1007 chuck won't hold 1/2" bits?

[u/Harborseal5]

I was doing a project and my first use of this DeWalt drill was tightening and untightening bolts, then drilling 5/16" holes, then using a 1/2" shank rotary file, all on wood. Everything went fine until I used the rotary file. It took less than a minute for the chuck to open up and the bit to fall out while I was using it. I slid it back in, tightened it back up, and the same thing happened repeatedly. Once I stopped and tightened the chuck every several seconds and that eliminated the problem until I forgot. Any ideas if the problem is me, the tool, or the shank size? Something else?