From my observations over the years - initials are pushed forward because the Subject has not been cleared and we need to see if there is anything we don’t know about them, but need to know.
I explain to industry managers when they worry about someone’s PR taking awhile that the wait is a good sign. In an area backed up with BIs, if someone submits a PR and the agents show up within a few weeks, that normally means someone either hit the jackpot or someone will be leaving the work group pretty soon.
Clearances don’t expire, the background information is reviewed to ensure there are no unreported changes in the Subject’s life. This is the basis of the continuous evaluation program.