i dont think it was either technique or strength, i think it was reaction time and agility. DDP was getting baited really hard by a simple jab, he had big reactions to 1-2 combo going straight back while Khamzat just instantly ducks and gets a double leg or both hands behind DDPs back which results in a trip or a throw to the ground. We saw DDP defend the first takedown a couple of times, but Khamzat simply either turned a corner and chained another, or just kept pushing and getting it that way over time.
I think one improvement he can make is to significantly lower his stance, the guy has a very sluggish stance where he just does not seem to have good balance, staying low and having a short boxing style might help him if they had a rematch. DDP just seemed like his body physique was not helping him in this fight, big guy, bulky, with poor side to side movement is what i think was his downfall, and im not sure he can really fix that over time.