shlucha, being mekasher to a Rebbe after he is deceased is not a Lubavitche thing. Yidden of all stripes and in all times davened and daven at the kevarim of our holy tzaddikim.
But the difference is that many Lubavitcher still think there Rebbe is alive often saying יחי אדוננו or יחי המלך.
Many Lubavitche think that the Rebbe runs the world.
Lubavitche have decided that there Rebbe is Moshiach.
These three ideologies all rolled into one are very similar to the cult of Christianity. Thinking that the Rebbe runs the world is pure apikorses.