If you pick a bunch of people who all think the same way it's not very surprising when a majority takes one particular side in a debate. For example, in another survey of the same group of experts, 98% agreed that the USA is better off after NAFTA[1]. That isn't to suggest they're wrong, but is does indicate that this might be a group of people who aren't very diverse in their opinions.

[1] http://www.igmchicago.org/surveys/free-trade