The cycle of a League of Legends patch is usually the following: the week before reaching the PBE, Riot's Head of Gameplay Design, Mark Yetter, gives a preview of which champions will be buffed, nerfed and adjusted. Then, a few days later, all the changes are fine-tuned. Once that happens, these changes are applied on the game's test server and are tested for the time remaining of that version on the PBE.
However, there are sometimes different adjustments that may come early, meaning more days are spent than usual on the PBE. This is the case for Amumu, who has received a very important change to his ultimate that, in principle, will not be applied in Patch 10.22.
Amumu's ultimate ability, as you all know, is a large AoE attack that immobilizes and disarms, while applying damage with AP scaling. Well, according to the changes in the PBE, the 'Curse of the Sad Mummy will now apply stun instead of immobilize and disarm.
This is a much requested change by the Amumu fan community, since despite the power of the ability, enemies could still apply spells and abilities. If you caught a marksman, perhaps this wasn't as relevant, but there are mages you don't want to be around for very long.
This reflects how old some champion designs are compared to newer champs. Amumu is simpler to play than almost anyone, and compared to Samira it's downright ridiculous on a conceptual level.
All things said, this is one more step to modernizing the champion before an eventual rework.