Challenger: Blew apart because the 'O' rings failed. This was predicted, but 'funding, politics and NASAs ego and complacency', launched the ship any way.
Columbia: Burned up on re-entry; again, due to complacency.
Yeah, ok. Here, a list of ten rocket accidents in case you think two is much.
https://www.popularmechanics.com/space/rockets/g18751736/rocket-launch-failures/
As you may notice, only one of them is from NASA.