I think it's because several celebs have used Jagger.
Lindsay Davenport has Jagger
Jonathan,
Brett Scallions [from the band Fuel] has a son name Jagger Song, and
Soleil Moon Frye has a daughter named Jagger
Joseph Blue.
I think it's more a celeb-influence name than a "omg i love mick jagger" name. Though I'm sure a few people used it because they wanted to honor
Mick.