Shorter barrels means the BBs spend less time in the confined space of the barrel and thus less time with backpressure behind it accelerating the BB to its ultimate destination, the crown of the barrel. The same principle applies to firearms - you do get velocity drops if you compare the same bullet weights/powder/primer loads to lesser barrel lengths.
The pitch change makes sense because as the weight of the BB increases there is greater backpressure, or blowback. In fact, I would posit that if you test with a longer barrel, you may actually see the opposite effect (even greater acceleration) because the backpressure is higher and the BB spends more time in the zone of acceleration, ie: the barrel.
I don't have stats on that, its more a rule of thumb, but the ballistics of real bullets are very well known and calculatable. In fact I have a program called Quickload, which calculates velocity, chamber pressure, etc and takes inputs like barrel length to do this.