Not sure if this has been mentioned yet, but a political analysis I've read recently is:
Buttigieg is growing because the establishment has sort of cancelled their support to Warren & Biden as they realized how little chances they actually have because of n factors and opted instead for this weird, almost unexperienced but relatively progressive and lacking a scandal in his biography, dude called Buttigieg.
Warren was never the "establishment candidate" but Biden - yeah... he appears to be done unless some miracle happens over the next couple of weeks. A while ago I thought those two would be frontrunners. Never would have pictured Butigieg up there, good for him.
If the establishment had their way Bernie wouldn't be at the top. We'll see after Super Tuesday.