Nate Silver defines herding as
the tendency of polling firms to produce results that closely match one another, especially toward the end of a campaign
On November 5 2012 Drew Linzer wrote a post "Pollsters may be herding", that stated:
[A] major concern heading into Election Day is the possibility that polling firms, out of fear of being wrong, are looking at the results of other published surveys and weighting or adjusting their own results to match. If pollsters are engaging in this sort of herding behavior – and, as a consequence, converging on the wrong estimates of public opinion – then there is danger of the polls becoming collectively biased.
I don't have a very strong reason for thinking that thread on Linzer's blog constitutes the origin of the term, but it's the earliest reference I could find. Some of the commenters on the thread asked about where this term had come from, and Linzer introduced the concept as though it was new and needed explanation.
In addition to who coined the term, I am also interested in when it was coined.