How? Human nature, and physical reality of the limitations on voting. Voters are less likely to notice nodes later in a conversation than at the top. And they are also less likely to have enough votes to vote for nodes near the bottom, having used up their votes before reading this far. If you have 3 votes left, and try to vote for 6 nodes in a thread, only the first three will be accepted - later nodes will get less votes because of this.

This applies whether you're reading nodes from first to last, or last to first, presumably if most people read nodes in a single direction, that's the direction with bias.

In general, the earlier one responds to a thread, the more likely that you will get a maximum number of votes, even if someone later in the thread has a "better" response.

That said, while I understand the human theory behind artist's statement, I highly disagree with the solution. I think that randomising anything would make threads impossible to follow, and that is a much worse thing, IMO, than the slight voting bias. It just means that part of XPWing is to "post early, post often". For those who aren't XPWs, then it's entirely irrelevant.