The previous respondents have come close to explaining the answer, but it’s not that the wheel is turning less than 50 rpm.
For example, assume a vehicle on 2’ diameter tires traveling at a speed of around 25 mph.
The circumference of the tire is 2 * pi * radius 1 = approximately 6.3 feet
The vehicle traveling 25 mph is traveling 2200 feet per minute. That translates to approximately 350 RPM for the tire, or almost 6 revolutions per second. If the camera shoots at 50 frames per second, then over the course of, say, 10 seconds you have 500 frames of film showing the tire rotating about 60 times.
What becomes obvious is that the rotating wheel will almost never synchronize with the film speed, so that the wheel will appear to be “advancing” with each successive frame. Depending on the synchronicity of the wheel and camera, it very well might seem that the wheel “advances” in the film (and you’d never notice an oddity if it did, because you expect that to be the case), but when the wheel’s rotation appears to lag because of the filming, and each new “snapshot” appears to be a little “less forward rotated” than it was for the last snapshot, then your brain interprets the wheel as rotating in reverse, and you certainly notice the discrepancy, when the vehicle moves forward.