I realize that this is speculation; however, from my FlightSafety - Raytheon days, I found that the average weight of that vintage Bonanza was roughly 2400 to 2500 pounds. Even at 2400 lbs, the aircraft would have been overweight (3,663 Max Ramp, 3,650 Max T/O).
Moreover, it will be interesting to find out where the CG was for this flight. Bonanza’s don’t like flying aft CG and/or overweight.
The other thing to consider is that looking at the registration records, this aircraft appears to have been fairly new to this pilot (within a year). That, in and of itself, is not an issue. However, (and I don’t remember if that serial number had it installed or not) I wonder if there was an issue with altitude compensating fuel pump. If installed, and the pilot leaned additionally, this would rob the engine of the necessary fuel to develop max power. When I was an instructor at FlightSafety teaching in the piston classes, this was an important topic of discussion with pilots – some who had never seen such a thing, and those who were used to it, but found themselves without it with their new Bonanza (TCM doesn’t offer that in the current iteration of IO-550 used in the Baron and Bonanza).
There is a lot more to this story that we don’t know about yet – it will be interesting to get all of the details.
Just my .02 cents worth