I could be wrong, but if the process is run too many times, PPM puts it onhold. I could be wrong, but I recall a 'rouge' process where this happened.
Otherwise, agree with maabhaven, and if the GEL step has had 'save and return' pressed (GEL script may not have been updated), the process will be put on-hold.
We have a data exception in PROD to alert when a Process is On-Hold, as sometimes when migrating GEL scripts to PROD, the process is not revalidated.