He blames food poisoning, according to the NFL.com report:

As for why the test came back dilute, Foster explained in detail. He said he was sick before the Combine. He was throwing up, had diarrhea, couldn’t keep anything down and was cramping. One adviser offered to have a doctor put him on IVs to hydrate, but he didn’t want that. He saw a doctor, got some medication and started hydrating himself.

Foster believes it was food poisoning.

“I couldn’t eat much, but I had to drink water and Gatorade,” said Foster, who began the process as the consensus top linebacker based on his game tape and measurables. “Then a few coaches said something about me being too light. And I’m a coach-pleaser. I don’t care what everybody thinks, but I care what coaches think. So I drank and ate as much as I could without throwing up. Then I went in there, drinking and drinking water, trying to flush out my system from whatever was making me sick and trying to keep my weight up and took the test.”