There haven't been any bundles by itch.io that were advertised in the forums for a while (as far as I know). I think they do sometimes collate bundles using Discord. 

When itch.io starts a bundle, an admin or moderator starts a thread, and people can submit their games in the thread.

Itch.io chooses the games from those submitted. The last time they included criteria of what they were looking for - such as games with the most or highest ratings and reviews.