If you mean Rubio-supporting areas, there’s no mystery. The largest employers around Des Moines are financial services, and those upscale suburbanites rejected Gov. Branstad’s pleas for Christie and followed the RNC’s push for Rubio. A lot of those financial services execs are young, and the older ones bring their offspring to the caucuses like they bring them to church and the country club.
What I'm saying is (historically) moderate counties went for Rubio and Cruz.
As I've been unable to find any factual data on crossovers, instead relying on exit polling and historical trends, I can't reply with 100% certainty. But, I will also note that if there was some type of crossover strategy, I don't think the Cruz campaign was involved, based upon the Cruz campaign spokesman's’ reaction one hour after Iowa voting had ended, where he was worried about big voter turnout favoring Trump.