I Googled a bit. This is possibly the most insightful take on it I've seen so far:
http://www.theguardian.com/technology/2011/oct/06/why-do-people-hate-apple
tl;dr - because Apple products tend to invoke emotion, that alienates those who aren't sharing that emotion, who then respond with equally emotional dislike of the company, the product, and you.
It's a pretty human answer, I think. It also rings true for a certain other interest of mine
I generally agree with that article. However it makes more sense to me the other way around. Apple customers become identified with their products because of the emotions they evoke - this leads them to hate whatever threatens their way of doing things. So, rather than those feeling "alienated" doing the hating, the hate stems from the reaction to defend what one is identified with.
Of course the hate from being "alienated" is also real, I just don't think it is that strong or frequent going in that direction.
Add to this, I think Apple's brand identity adds significantly to how emotionally attached their customers become to the products. It's not just the experience of using the products themselves. Included in this is the illustrious history of Apple as well as their standout personalities (Jobs, Ive, etc.). Apple is a very clearly defined group to belong to, and the company certainly represents success and strength, so this offers a sense of protection.
So, Apple customers are emotionally identified with the products, and with Apple as an entity itself. This leads to Android hate. (And probably in a lesser degree other platforms too, like WP, but they are of course not as big an "enemy".) And this I am sure is just as strong and common as hate for Apple.
On the Android side however, I think the emotional attachment arises from something more ideological: the concept of freedom. Freedom to customise your phone, the OS itself, or the freedom simply to choose from so many different products. With the emotional attachment to the actual device user experience being less strong than on the Apple side.
And I am 100% sure the hate flows just as strongly in both directions. You only hate as much as you are being hated. It's just that the hate for Android is less focused since there are so many manufacturers on the that side. There is "Samsung hate", "Google hate", etc. So it doesn't appear as striking as the hate for Apple.