How often is a bad review from the reviewers due to the potential competition with your work? Reviewers are expert in their fields who can understand your work but are also potential competitors if they worked in the same field as you do. How frequent is a bad review from the reviewers due to the potential competition with your work so they can stall your publication?

For example, if you were a reviewer, would you be excited to learn that I have published research that are superior to yours or prove that your research direction was wrong? Another extreme example, if you were an HIV researcher/reviewer, how excited/horrible would you feel if I publish a cure for HIV and will be nominated for Nobel prize tomorrow and you will be sitting down there listening to my talk?  What incentive do reviewers have to give out good/fair review?
In my experience:

If the editor is fair, it doesn't matter terribly much.

The decision to reject a paper is not taken by the reviewers.  The decision is taken by the editor. The role of the reviewers is to give a recommendation, upon which the editor takes a decision.  If a reviewer recommends rejection (or acceptance, for that matter), the editor is free to ignore this recommendation (this has happened to yours truly).  If the editor suspects that a reviewer recommends rejection because of potential competition, or for any other improper reason, then the editor should not weigh this recommendation heavily.  This is one of the reasons why there should always several reviewers — at least in my field (atmospheric remote sensing), I've only come across cases with two or three reviewers.

Secondly: The editor knows who the reviewer is.  If a scientist makes a habit of recommending rejections for improper reasons, he or she will get a reputation among editors.  Editors are usually well-known colleagues in the field, so you really don't want to be known as the envious guy (or gal) who will reject major works of research because it was not invented here.  In the best case, they won't be asked as reviewers any more.  In the worst case, it may harm a scientific career.

Perhaps the scenario described above is somewhat naive, but in those cases where I've either been a reviewer, or been corresponding author, it would apply.  The same for open review papers (such as the geophysical journal, published by EGU, Atmospheric Chemistry and Physics).  It might be different in other fields, this I don't know.  I hope that even in other fields, the editor does more than copy-paste and blindly follow whatever reviewers recommend.