I have to check my literature on this. However, I believe that re-ingestion is one of the things influencing this. As speed reduces the reverse thrust plume re-ingests into the inlet affecting the efficiency of the thrust reversers. Reverse thrust decreases with decreasing speed.
Will have to check this when I am back in the office.
I think you will find that the
reingestion is at a cross-over point and not linear until then. Affected by engine power setting (air out the reversers) vs forward speed.
Just above no-effect,
below a chance for some ingestion mostly through the fan (FOD) and increasing chance for some in the core and a surge