“People who don’t understand that there are so many different ways in Avodas Hashem are just stupid.”
It might be the reason they refer to certain people as “OTD”. Either way, since different people have different ideas when they talk about “the derech”, those same people have different ideas about what it means to be off. Which in turn leads to different answers to the question, why they go OTD.
To echo the sentiment of FeifUn (without taking this thread in the direction of whether one should or should use said eruvin). Have you ever seen the pamphlets for and against the eruvin in brooklyn?