The forum is meant to do that, each alert is only meant to be kept for 3 days.
Problem is the process to go round deleting those old ones cant be completed in the time necessary so manual wiping is required ocassionally and thats only possible with a complete wipe.
Today wasn't planned given it was done only a few days ago but a server node was having issues and things needed resetting and wiping alerts is part of the general clean up.