I speculate that re-posts get voted to the front page because the users who are voting on them didn't see them the first time around.

The fraction of HN users that see a post on average depends on how long a typical post stays on the front page and on how often a typical HN user checks HN. I don't know what those parameters are, but I think it's plausible that a typical HN user sees fewer than half of all front-page posts, in which case it wouldn't be surprising for there to be some re-posts.

If something changed about those statistics--for example, perhaps more posts per hour leads to a typical story staying on the front page for a shorter time--then that could account for an increase in the number of re-posts.

Also, I'm not sure that there is actually a trend towards more re-posts on HN. It seems like the kind of thing that would be easy to mis-estimate.