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Unless it was deflating thru an orifice, falling and/or flying in a vacum, it was NOT gaining momentum.
After pondering my above comment, let me clarify my main point, once the tire left the car, it would unlikely ever gain energy, it could store energy in compressing the inflated tire when it struck a solid object, but most of that would be released on the rebound, it might change direction, and it would change velocity as gravity effected it, but eventually thru friction, air drag, energy absorbed even by thought to be "solid" objects, it would eventually come to rest. However if the wheel was accelerated to very high speeds/rpms thru slippage with the track or the fact that tire was in the air before seperation, it could easily increase is ground speed after seperation as the rotational energy is converted to velocity as the loose tire gets traction, hence the appearance of gaining "momentum", which really is just a conversion of rotational inertia to ground speed. Which is also why so much harm can from a low speed wheel seperation.
Reducing chances of wheel seperation likely should be the focus of prevention IMO.
To carry these thoughts even further, what we may be seeing as well is the effects of gyroscopic precession. It appears in the video the wheel was spinning at a significant speed well past separation from the axle hub. I've seen aircraft gyro's literally tear themselves from their mounts in an event called gimbal lock. This can be a very violent event and some of the tires reactions to each hit (bounce) may have induced a more violent reaction/response by the tire (especially when you see it bouncing around the trucks). This may be what we are seeing vs. it appearing to gain velocity. 
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