What time of day was this being cut (early morning, mid morning, or afternoon), was that moisture or dust being discharged? This appears in the video to be what I consider a flat cutting area (correct me if I'm wrong), and I could hear the rpms dropping when you would first push the sticks full forward. I would not think the hydraulics would pull the big Vanguard in that manner, can you explain what may cause this, it could very well be you are on more of an incline that what appears, and throwing the shorter cutting into the equation may add up to the drop. Then there is always that chance the mower sounds different when being heard outside of sitting in the seat when operating. This appears to be grass that is not growing very fast, and you are taking a small amount off, but while cutting at a lower height, is this correct? What types of grasses are on this property, you appear to have some grass that looks more brown as if it was trying to go, or already has gone dormant, as would Bermuda in out area once the days become shorter and nights cooler?