When we create pull requests, the default repo is the parent repo from which we are forked from.
i.e. Repo A, and Repo B is a fork of A.
I create a pull request in Repo B, it sets up a default PR aimed at Repo A. The description field contains a diff between the two repositories (which you can imagine is a massive list) and this opens up opportunities for mistakes.
I don't see any options within Bitbucket to change the target.