This isn't really a surprise, since Ben Carson makes Republicans feel good about themselves, validating their sense of themselves as fair, not racist -- "See, we like him and he's black!":

West Long Branch, NJ – The Monmouth University Poll of likely Iowa Republican caucusgoers finds Ben Carson and Donald Trump tied for the top spot. This marks the first time since July 26 that a poll in any of the first four nominating states has not shown Trump with a nominal lead. Not surprisingly, given the top two contenders in the poll, most Iowa Republicans prefer someone without a traditional political pedigree. At this early stage, though, the vast majority of voters say their eventual support could go to one of several other candidates in spite of their current preference.

When Iowa Republicans are asked who they would support in their local caucus, Ben Carson
(23%) and Donald Trump (23%) tie for the top spot. The next tier of candidates includes Carly Fiorina (10%) and Ted Cruz (9%), followed by Scott Walker (7%), Jeb Bush (5%), John Kasich (4%), Marco Rubio (4%), and Rand Paul (3%). The last two Iowa caucus victors, Mike Huckabee and Rick Santorum, each garner 2% of the vote. None of the other six candidates included in the poll register more than 1% support.

“These results mark a significant shake-up in the leaderboard from Monmouth’s Iowa poll taken before the first debate,” said Patrick Murray, director of the independent Monmouth University Polling Institute in West Long Branch, NJ. “Carson and, to a lesser extent, Fiorina have surged, while Walker has faded into the background.”