I was flight lead on an MD-80 series commercial airline trainer a long while ago. Unless there has been a mod since then, the reverser deployment command will not go true unless a main landing gear squat switch is made. In the video, the left main gear is seen to touch down briefly and the reversers deploy shortly after that. It appears to me that the reverser levers must have been in the reverse position, and the first bounce enabled the deployment. A lower order possibility is that one of the main landing gear squat switches was failed closed.
In any case, airborne deployment of the reversers is not a normal operation for the MD-80.
It is possible for the lower reverser bucket to strike the ground if the deployment sequence occurs with the main gear on the ground at higher pitch angles where the nose gear is not on the ground - adding code to detect this was the last change made before the trainer received FAA certification.