I can’t speculate why this happened but I do know that I don’t let mine get below 1/2. I was in NMex on a trip and was driving on a rough side road with my DEF sitting at just under 1/4. Got an error code and slipped into limp mode. The truck was new so I took it to a local dealer on a Sat. The place was a zoo and wouldn’t be able to get to it til Monday. But, he asked if he could read the code. It was a DEF squirt error, missed one I guess. The he asked me if I wanted to erase the code. Uhhhhhh, YES!. Filled up the DEF and the code never came back. The rough road must’ve caused the DEF to slosh and miss a cycle. IMO, the whole DEF and EGR setup is made to fail. It’d be nice if we could do like the Canadians and delete this stuff forever. Isn’t it a huge penalty to be towing something and your truck is going to limit you to 50mph because of the DEF? On a major highway that’s a real safety concern.