Well, there's a big brand name I didn't expect to be sold, but I suppose if they'd already been acquired by a business that doesn't play in the same direct space, then a disposal was always a possibility.
You rather imagine it was cost effective for Sonova to acquire them, delve into all of the Sennheiser R&D and then sell it on when that source of info was exhausted. Looks like they were acquired for €200m and Sonova generate far, far more through their hearing aid division (think 15+x that outlay).