Similarities of Pixar and Dreamworks: any published explanation?

To be honest I don't think there is any real issue here regarding similar films from the two animation stables and disagree with anyone who thinks there are more than two comparable sets of films in the original question.

Firstly, we can eliminate two of your examples, as Dreamworks were not involved (The Wild, Robots), so perhaps the question could be made more general - this then negates the alleged 'conflict'. Also, Flushed Away was an Aardman production before Dreamworks came on board as a distribution partner. In fact, the only films that show any resemblance to each other are A Bug's Life (which was a remake of Seven Samurai) and Antz (which was an semi-original story), and Finding Nemo and Shark Tale, which share nothing else other than being set under the sea.

Is it well known that there are occasional years when live action releases seem to follow a similar theme (Armageddon/Deep Impact, Volcano/Devil's Peak, Snow White and the Huntsman/Mirror, Mirror, even this year with Olympus has Fallen/White House Down). In most cases this is down to coincidence, or one production company rushing to get their version out before the other, thus affecting the release dates.

Pixar and Dreamworks have no reason to copy each other. The amount of time and money invested in any given animated feature is phenomenal and both companies know that merely copying the other (or even entertaining the fake notion that there is a rivalry between the two houses) does them no favors.

In fact, Pixar went so far as to scrap one of their future releases, Newt, due to possible conflicts with other productions. Some industry insiders believe it was the similarity of the story to Blue Sky's Rio, others attribute it to the success of Rango (ILM) with Pixar not wanting to turn out 'another' lizard/amphibian flick. You can see some of the concept art for Newt at the Pixar Wiki page.

Bottom line: the similarities are few and far between and these can be put down to coincidence.

Adding to Christian Rau's excellent answer, I would also like to say that modern feature films are rarely the creative brainchild of a single person, but rather created with a lot of corporate input, often from marketing departments backed by data from polls and focus groups.

"Robots are going to be hot this year!", they'll exclaim. "Make us a flick about robots!". And perhaps a preliminary script about a robot in a post-industrial wasteland gets taken out of the pile and prioritized at Pixar, while a similar premise that was on the backburner at Dreamworks gets similarly pushed ahead.

All movie studios, and especially ones like Pixar and Dreamworks that share a market, are affected by that market, often in similar ways.

I cannot provide you with any interview, but only common sense. Those similarities are observable quite often and even more than the movies using similar concepts, they are usually also released in close succession (that's why I don't think the example Wall-E - Robots is a good one, given how far both movies are apart in their premises and release dates, but there are indeed many good examples).

That's why I have always interpreted this as a mere competition between two major companies. One of them releases a movie and immediately the other one has to release a similar movie to compete with them or to use the success of the previous movie for driving the success of their one movie. Or maybe one of them hears that the other one is working on a movie about something and immediately steals the idea to release a similar movie at the same time. The industry is full of such examples of companies competing in this way and the usual similarities between Pixar and Dreamworks movies released at roughly the same time is a very prominent one.

There is only one movie pair in this list that is a real and proved issue.

A Bug's Life and Ants.

As you can read here, and in more detail in Steve Jobs's biography, it really seems like there was something going on at the time between Pixar and Dreamworks, and in particular between Dreamwork's Jeffrey Katzenberg and Pixar's Steve Jobs & John Lasseter.

For every other movie pair in the list I agree with Christian Rau as to their being results of a trend and a rush for competition.