In an earlier post (April 16, 2013), the contemporary substitution of multiple for many was attributed to iconicity. However, on second thought, there is an alternate explanation.
Multiple may have arisen as a designator of number because it is non-committal as to whether it means ‘many’ or ‘several’. This new meaning––something between the two––evidently fits a semantic niche that speakers and writers wish to exploit when neither many nor several fills the bill. This new connotation of multiple is: ‘not as few in number as several and not as great as many’.