What's the reason for this frog finger shaped parking lots at Köln/Bonn airport (CGN)?
@Simon is right. The 'parking lots' are actually dispersal areas in the (previous) military airfield at that place. 

The Cologne Bonn Airport started out as an airfield for Luftwaffe in 1938 in what was a former artillery shooting range. After war, RAF took over the airfield, with the first civilian flights in September 1950. The airfield was converted for full civilian use only in 1957.



"Koeln-Bonn-Airport14" by Borsi112 - Own work. Licensed under CC BY-SA 3.0 via Commons.

The original 1886m (32L/14R) runway is near the bottom of the image (near the dispersal area). The top runway, 3800m in length (32R/14L), was opened in 1961.

In military sirfileds, the parked aircrafts are usually dispersed in order to reduce vulnerability in case of an air raid. According to US Army Field Manual 5-430-00-2 (accessed via globalsecurity.org)


  If dispersal of aircraft is possible and consistent with active defense measures, varied parking patterns provide fewer lucrative targets for indirect-fire weapons. 


It also gives some (conceptual) layouts for aircraft dispersal:



Source: globalsecurity.org

The aircraft are parked in such a way as to reduce the probability that more than one aircraft is hit simultaneously. These dispersal areas have banks of earth or small walls around them to reduce the aircrafts’ susceptibility to the effects of bombs landing nearby and to reduce collateral damage in case one of them blow up. It might've looked like this:



Source: portalnews.gq