Data published by The Upshot point to other possible swing group in open primary states: people who call themselves Republicans but are registered Democrats. Those people seem to be strong Trump supporters, and they could be showing up along with other Trump-loving Democrats in certain states.
Once again, however, the exit polls are hard to read on that issue. Only about 5 percent of voters in open primaries admit to pollsters that they're Democrats, and usually that's too small a portion to break out by candidate. In Ohio, about 8 percent of Republican primary voters were Democrats, and 39 percent said they supported Trump. That's more support than he got from Republicans (36 percent) or independents (35 percent), so it's possible that Trump won a few percentage points from enthusiastic Democrats in other open primary states as well.
Trump certainly does seem to be bringing some new faces into our political process. He received 30 percent of the vote from newcomers to the Iowa caucus and 38 percent from first-time primary voters in New Hampshire. Reuters polling over the last six months of 2015 found that about 27 percent of "very motivated" Republican and independent voters who have not voted in the last four national elections supported Trump, compared to about 23 percent of people who had voted before.
It's certainly possible some of those new voters are finding it easier to participate in primary elections in the open states, even if the mechanism isn't entirely clear. Trump's strength in open states is probably derived from some combination of those voting groups, and we'll have to wait to see if the pattern continues in the remaining primaries. Utah and Arizona will both hold closed primaries next week.