Alright, well I guess he started it off.
Basically they're a "religion" that was founded by a science fiction writer who slowly and eventually manipulated those who were followers of his "religion" into being extorted out of their money through the use of brainwashing. Blackmail is used to keep those who were in the church and are now against it quiet. Human rights violations have been taken against some of those who speak out against them. They also bait people into their "religion" by telling them lies about what they're about, and then when people are further in, they change what they're about.
Basically, what we're protesting is that their actions make them against free speech... they are a criminal cult that extorts money out of its followers... and their tax exemption status is fradulent (as it is not a church). We have nothing against $cientologists, but rather more with the criminal institution governing it.
Keep in mind, this is a nutshell. It is best that you research it more if you're interested in learning both sides.
I might have gotten some of this wrong / used poor wording. Please correct me if this is the case, I don't want to tell something overtly false.