About sector expire

After a farm is 100% plotted, some of the sectors will become expired from time to time. What’s the reason of this?

The blockchain might generate two valid blocks at the same time, and this forks the blockchain. The final decision is to keep the longest chain and obsolete the short one. Is this the reason of sector expiration - the sectors consists the short branch are removed?

I believe it is already answered in What triggers a sector to start replotting