Over the past year or two I've noticed calls for submissions from major publishers such as this one:
being passed around the social networking sphere. In your opinion, might this be an attempt to cut agents out of the picture or is it perhaps a knee-jerk reaction to the changing state of legacy publishing? Or something else?
I'm going with your answer (c) - 'something else'. And that something else is 'trying to find good manuscripts'. Publishers know that agents can't possibly find all the good stuff - although they do like us to do that, as it helps cut down on their own slushpile reading. We all get sent far more submissions than we can handle, and agents tend not to have the resources to read absolutely every submission in the country. So if the publishers have the resources to put towards reading submissions, they will. Not all of them do; some of them do some of the time. Some publishers always have submissions open and some will never have them open. It's nothing new, it's just that it's easier for word to spread when they announce online that submissions are open. In the olden days you had to phone their switchboards to find that out.
Publishers and agents aren't enemies, by the way - we're all working towards the same goal, of seeing great books by great writers out in the world - so it's unlikely any agent would see this sort of news as an aggressive gesture by a publishing company. And if they did, well, that's their business. Ya can't please all of the people all of the time.