1. The poll was framed in a way that did not make my opinion
available.
2. Nobody else had expressed my position so I thought that it
was uncommon.
3. I assumed the poll was meant to try to force Dropbox to drop
her from the board.
4. I expressed a position that wasn't that political but that
I knew would anger or confuse political witchhunters.
5. It is quite common for people to downvote information that
does not agree with their ethical aesthetics.
I think those are good reasons and I think they're quite common. However if you think you can argue beyond a mouse-click go ahead.