Landmark is a self help community which teaches people to teach responsibility for their life. That means that the participants might weaken ties with family members with people who abuse them or demand to be treated differently.
That's enough for people to come and call a self help group a cult.
Then Landmark exerts pressure on participants into recommend Landmark to their friends. It's something that ethically questionable and it provides further ground to speak of Landmark as a cult.
It however doesn't mean that Landmark isn't effective in improving the life of the participants.