Why did the editor give my peer review to another peer reviewer before their review was written? When the editor of a paper I recently reviewed circulated the final decision letter to myself and the other reviewer, I was surprised to see that the other reviewer had apparently had access to my peer review before submitting hers. This was clear because her review explicitly referred to comments made "by the other reviewer". Note that this was definitely another peer reviewer, not the editor. 

I've never been given access to another reviewer's comments before submitting my own, and I'm curious as to why an editor might choose to do this. Don't editors typically want the reviewers to be independent of one another?
This depends a bit on the journal but is not that uncommon.  

If a paper is resubmitted after a first round of reviews, then the reviewers at the second round can sometimes access the comments from the original submission.  In particular, this allows the reviewers of the updated version to check how the authors have accommodated the comments of the original reviewers.

It can also happens that a second referee report was never submitted, or that some glitch has delayed the refereeing process; as a result the editor has to find a trusted referee to quickly submit a second report, and this trusted referee can be given access to earlier reviews to speed up her/his decision.

It may also happen that the original reports of two referees disagree in their recommendation, and that the editor sends all the relevant material, including referee reports, to an adjudicating referee.