I'm not sure if this is a legitimate question to ask,but I noticed this sound change in a few germanic languages, such as Old English and German. How did it happen?
This is one form of palatalization, a very common process across languages.
Typically, /k/ fronts and spirantizes to /ç/. In isolation it often goes further, to /tʃ/, but after /s/, /sç/ may coalesce to /ʃ/.
Consider, in particular, Italian, where an original Latin 'c' (/k/) before a front vowel has usually become /tʃ/ ('ci'), but after 's' it has become /ʃ/ ('sci').