Identification. They are so identified with Android that they take criticism towards "their" OS as a personal attack against themselves as persons. This is a psychological mechanism that often begins with having some insecurity in one's personal emotional life. So you "team up" with something strong, in this case the total number of Android users as a group, backed by a powerful corp like Google - in order to feel stronger and more safe, to feel that one belongs to a strong group. This is a well known survival mechanism.
With iOS being the main competitor to Android, then, Apple is perceived as quite literally the enemy. In extreme cases this grows to actual hate, for those with a particularly strong feeling of insecurity. It's much the same thing as football fans being prepared to use violence against fans of another team, or nationalists wanting to kill citizens of another country based on no rational reason at all. Quite common obviously, and always disturbing at any level.
"Fandroids" hating "iSheep" or vice versa, it's all the same and goes equally both ways. It is at this level that any interesting, constructive discussion in the forums comes to a halt, and petty bickering takes over. The remedy is to notice the anger when it arises, take a moment to look at it and realise that this is after all only about who prefers which phone or OS, and just let it go.