Strut mount

I have replaced the upper strut mounts over the weekend. Two days later I checked and the new mounts have already severe cracks in the rubber, actually worse than the old once. The strut mounts were purchased from MB. There was no jaming, forcing or bending of anything when I installed them. The crack in the rubber on the left side strut mount is a horizontal crack almost 2" long in the direction of the car, engine side, and the distance between the upper portion and lower portion of the rubber is almost 1/16" I took a small kitchen knife and I could push the blade into the crack.
Has anybody experience that or does anybody know why this happened.

Thank you all in advance.