So, in the night from May 30th to 31st (CET) Valve has deleted 28 games from the German Steam Store. Most of these games - not all - have been indexed by the German "Federal Review Board for Media Harmful to Minors" (BPjM) and some have been on the index list for over 20 years (25 years is the maximum).
The deletion was first reported on by Steam Community Group forUncut! who "live blogged" the whole thing while the keys were being edited (see full list there). They searched SteamDB.info for the "PurchaseRestrictedCountries" tag of games, and the games which were locked for Germany just kept piling up.
This happened to fairly old games like the Postal series, Bloodrayne, AvP Classic, Painkiller, etc. These games could prior be bought in the German Steam Store, although being "indexed" means games cannot be actively advertised for or sold without proper age verification (technical measures - age gates don't suffice). As to why they were taken down now and not earlier, there are only speculations, as Valve has not come forth to shed light on the proceedings.
Users of the group are speculating that the games might have "accidentally" been published for Germany in the past with Valve only now realising their mistake, others say it might have been due to intervention of the BPjM or the KJM (a federal youth protection supervisory board for broadcasting systems) or even a competitor issuing a cease and desist, as EA and Ubisoft have implemented crude youth protection measures in their respective platforms (time dependant selling of mature rated games or a labelling for youth protection filtering software). As it is, Valve is still in violation of German Youth protection laws by selling USK rated 16/18 games to anyone without proper prior age verification system. In the Steam SSA it is stated that anyone of age 13 and up may freely buy anything from the platform, while there is not real age verification other than an age gate.
Main source: http://steamcommunity.com/groups/foruncut#announcements/detail/604985754884746774
other sources: http://www.neowin.net/news/27-games-get-pulled-from-steam-in-germany-likely-for-being-controversial, http://techraptor.net/content/26-controversial-games-pulled-steam-german-crack