How are copyright transfers for a journal article done? Almost a year back, I have submitted a manuscript for a Journal as a research article, and after almost 11 months and after undergoing two rounds of peer review and referee suggested revisions, I finally got an acceptance letter. After getting acceptance, I thought I would receive a letter to sign a copyright agreement and asking my permission to transfer the rights for publication, but to my surprise I did not get any such letter. I then read the copyright document on the Journal's website (under instructions for authors segment, one is supposed to read even before submitting a manuscript) and found that as per the document the copyrights laws are automatically transferred once the manuscript gets accepted, and come into force from the day of getting the acceptance letter. Is this quite a common thing with Journals. The Journal is although not a top journal but fairly reasonable and from a fairly known publisher and I am sure its not a fake or predatory. There are no charges for authors whats so ever.

Question : Can the copyrights be transferred without the author explicitly signing (electronically or otherwise) any document to that effect?
There are very few kinds of contracts that require a full written form and a signature (depends on the legal system, but I think in most places real estate is the common exception). It's just easier to prove the existence of a written contract than an oral agreement if things go to court. 

But in this case there seems to be no need. You said yourself, that the terms in the instructions you are supposed to read before submitting, so you can hardly argue to have missed them. You also cannot really argue that the terms are unreasonable or unexpected, since a copyright transfer of some form is to be expected when publishing.

Legally what happened is that the publisher offers a contract that involves transfer of copyright in case of acceptance and you agreed to that contract by submitting.