Looking into it there are a few things we know.
The reason the cops were there was a call about shots being fired.
When they showed up and were near the RV, they heard what sounded like a recording saying the bomb was going to explode in 15 minutes.
They started evacuating anyone nearby and called the bomb squad but then it exploded.
They found human tissue in the wreckage, likely someone was at the epicenter of the explosion.
Best guesses?
My first theory is that the dude was planning to bomb somewhere in the city, but was simply lingering, hoping for a bigger crowd at some point between Christmas and New Years. They have not told us who called in the "shots fired" call or how many people did, so my guess is someone knew what he was planning and where he was staying and called in the tip to get police there, hoping to spook him to back out of it. Instead, the dude freaked out and thought they were on to him, gave them the bomb warning out of desperation while acting like a recoding, then, realizing there was little way he can get out of the situation, decided to just commit suicide then and there with the explosive.
My second theory is a bit more dark, but basically, someone was assassinated. Why this theory? Well, the original call was for shots being fired, and assuming the call was genuine, that means someone shot someone else in the area. Whoever it was either pre-set the RV to explode, or brought an explosive with them, did the deed to make sure the victim couldn't escape, then left the bomb on a timer with enough time for the killer to fly away into the shadows, using the recorded message as a way to keep others from getting too close. Once the bomb went off, it basically scorched any forensic evidence and blew up the victim into nothing but scattered tissue and bone, making identifying and investigating everyone extremely difficult.
Could there be other options? Of course, I ain't a detective, but these two seem like the most likely to be the cause based on what I know.