He over-rotated on the barrel roll. As he unintentionally came onto his back, his legs were likely open and uneven. That probably caused the flat spin. Jumping an S-Wing at 14 wingsuit jumps didn't help any either.
The reserve pilot chute hesitation could have been due to the slow post-cutaway speed and the burble of the larger suit, although there may have been a gear issue too. IMO it's best, during skydives at least, to minimize the burble during deployment by closing the arm and leg wings.
In umpteen hundred wingsuit jumps, I've had 1 experience where the final locking stows of my main wouldn't come undone, leaving me with the bag towing behind. This cleared as soon as I looked over my shoulder - the air rushing past me at that point was enough to finish off the job and I had only a line twist as a consequence (result of going asymmetric at that point). It's quite reasonable for either a bag or reserve PC in tow to be experienced for a few seconds in a wingsuit.