This behavior doesn't make sense to me. The allocation
of processors happens when LAMMPS is launched, before
the code knows what potential will be used. That part
is done by your MPI installation. So by the time
the script gets to a Tersoff or ReaxFF command, you
already have processors. I would try creating a simple
box of atoms at the start of your script - that will print
out how processors are assigned to the box. There is
no way that is affected by a later Tersoff/ReaxFF command.
Steve