This has only happened since switching to the new client and the most recent patch. The wifi I play on occasionally has downtime, and when this downtime occurs while I have the client open (more specifically when in a loading screen or in-game, achieving the "trying to reconnect" message), restarting the client causes League to downgrade the patch and re-download it.
Huge hassle for anyone and especially those with a limited internet plan. My speculation is that League seems to think the disconnection meets the conditions for a patching error/corrupted file, and so tries to re-download to repair.
Please fix and thank you!