There is (was?) an option to set the number of days to keep incomplete files. See if yours was set for 365 days--that might explain the annual purge.

However, old incomplete files are not likely to resume, so they usually end up just wasting space.

Incompletes are useless without the same downloads.dat and .bak files that are in the same folder. So, if you want to retry an old incomplete folder, just replace your current incomplete folder with the old.