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?
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1It's happened in Norwegian etc as well, but only in certain phonemic contexts – OmarL Mar 5 '19 at 15:57
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').
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2An interesting case is Venetian, where some words that in Italian have retained /sk/ have /stʃ/ instead, like s'ciao /stʃao/, Italian schiavo /skjavo/ 'servant, slave' from which the salutation ciao /tʃao/ (originally 'I am your servant'='at your service') – Denis Nardin Mar 5 '19 at 21:14