Sign copyright agreement before paper acceptance?
This is rather normal for non open-access articles in my field (Physics), however all journals I have submitted to until now had a clause that the copyright transfer is voided if the paper is not accepted or withdrawn. For example IOP’s copyright agreement contains the following sentence:


  In the event that the Article is withdrawn prior to acceptance, or is rejected, this agreement shall have no effect and no party shall be bound by it.


If such conditions are included, requesting such an agreement at an early stage seems somewhat reasonable to me, since it would be a waste of time to review your paper if you did not agree with these terms.

If such conditions are not included, this is a clear warning sign that the publisher may be up to nothing good. However, even with such a clause, problems may arise, as the publisher may just instantly accept your article.

For open-access articles (pay-to-publish), a similar situation holds, however, the copyright agreement usally requires less commitment from you, as you should retain most rights on your article (since the journal does not make money with being the only one making your article available). This not applying to the copyright agreement is another warning sign of a predatory publisher.

In both cases, you should read the copyright agreement carefully and ensure the reputability of the publisher.