I was curious about it, but from Reddit discussion, apparently this is totally normal; "Yes, the engines have been upgraded many times. To avoid having to recalibrate the software each time engines are upgraded, they are simply rated at over 100 percent thrust over the first engine's thrust."
Another interesting fact. The pad is NASA's. So you can imagine the crew that maintained it are pretty disappointed.... "Look at what you did to my pad!?!" The fireball was enormous. The view from the Cessna incredible.
That must have been just about a worst case scenario for the pad to sustain such a direct hit. Hopefully it was the strict safety protocols and not just dumb luck which kept everyone safe. That's another aspect of this event which can be studied and reported, so at least we maximize the take-aways. Track the entire response from T-6.
Apparently the rocket is designed to do that little lateral kick that you can see in the video and initially seems odd. 'Why is the rocket not going straight up' causes a little unease at first but apparently is nominal. If that's the case, then the time of the explosion may correspond to a point where they kick up engine output. It also may have been a heavier payload than usual requiring higher engine output? (Unconfirmed)