After his brutal defeat on December 10, 2011 against Igor Pokrajac (K.O. in 35 seconds), Krzysztof was thinking of a last fight in the UFC before retiring. After all, no one wants to finish his career on a defeat.
Dana White had provided him a fight, but a few weeks before his fight Soszynski felt that he could never again step foot in an Octagon, with particularly large losses of memory.
For more details on this decision, see this interview very interesting with him while it makes account of the reasons which it were pushed to leave the sport. One aspect of the sport which we hear little about.