This is what we'd expect to see if people who reply early were overwhelmingly more likely to give the first answer. It's also what we would see if someone did not like the way the poll was going and decided to rig it.

There is also the third alternative of a great comment defending option two showing up (or having been up-voted enough) at the time you mentioned, to sway "public opinion" in its direction. It seems highly likely that people would read the most visible comments (and be persuaded by them) before voting.

Now, I don't know which comment was the most visible at (or right after) 2013-04-20T20:00, but it looks like PhilipL's and buybuydandavis' comments are the most probably candidates given their current karma scores. They are also a defense of the second option (or at least closer to the second than to the first).