Actually, I'm pretty sure that bullet was yawing....
IIRC, the .50 BMG goes subsonic at about 2100-2200 yards (someone correct me on this if I'm off base on this, I didn't run ballistics...)
Even if the bullet is relatively stable transonic, doesn't seem unreasonable it would be tumbling at that range (?)
Edit...
Couldn't resist...
JBM says that the 750 grain Amax at 2800 fps MV goes subsonic between 2500-2600 yards... at 3000 yards it's right at 1000 fps.
The Amax is pretty much top of the heap with a 1.05 BC, but who knows what these guys were shooting...and at what elevation...