Looking at the Strava posts for 2nd place finisher Eddie Owens and 3rd place finisher Remi Leroux and comparing their splits to David's here's how it appears:
By aid station 1 at mile 8.5, the lead pack was six strong. During the next climb, winner Eli Hemming made a move around mile 10.5 and David tired to hold on, while Eddie and Remi let them go. Runners 5 and 6 also fell behind on this climb.
David held on to Eli past the aid station at mile 14.3, but Eli dropped David during the next climb around miles 15-16 (which, by the way, is after David claimed to "roll his ankle" - guess we wasn't so "hurt" to DNF at aid station 2 when it looked like he was still in the race). On this third climb, David appears spent - he had a 30-60 second lead on Eddie and Remi, but they slowly worked up to him and passed him during this climb.
By mile 17.5, Eddie and Remi were long gone and, with them, David's chances of podiuming. David kept running, albeit at a 6:20-7:00 GAP pace until he got to aid station 3 at mile 23.2. For contrast, Eddie ran those sections a at 6:00-6:20 GAP pace.
Thereafter, David DNF'd at aid station 3. He couldn't podium, so he decided to pack it up and take his toys home.
Again, no evidence from his running that he was hurt - just tired from pushing past his limits and blowing up. Hubris will do that to you....