This is a good explanation about why people defend brands, but in case of Apple, my experience is that the anti-Apple crowd is more emotionally invested in being anti-Apple than the fans are for it. It used to be the other way around when Apple was the underdog, but after the iPhone took over and became a sort of default for certain regions and social circles, the most loyal brand warriors are the anything-but-Apple fans.
The most vocal, emotionally invested anti-apple crowd are those who were once emotionally invested in Apple. Similar to an anti-smoker, ex-cult member, ex-mormon or vegan convert - those who have been burnt are often the loudest. The same psychological effects are in play - people will try to defend the hurt to their ego.
There is no brand of "anti-Apple" - that doesn't really exist as brands don't work like that BUT groups of people do though, and people can have identities of being in a group. (I can't really say I have noticed an anti-apple subreddit either)
One way groups defend themselves is to define the opposition - by defining the opposition to Apple fans as anti-apple-groups, it helps bolster the coherence of the Apple fan group and defend the brand and investment for the individual. We are under attack, close ranks and protect each other.
