C originally had =+ and =- (upto and including Unix V6) - they were ambiguous (a=-b means a= -b? or a = a-b?) and replaced by +=/-=

The original structs were pretty bad too - field names had their own address space and could sort of be used with any pointer which sort of allowed you to make tacky unions) we didn't get a real type system until the late 80s

ALGOL 68 had "=" for equality and ":=" for assignment, like ALGOL 60.

Therefore the operation with assignment operators were like "+:=".

The initial syntax of C was indeed weird and it was caused by the way how their original parser in their first C compiler happened to be written and rewritten, the later form of the assignment operators was closer to their source from ALGOL 68.