Yeah this is common. It's kind of a pain with us, because we have such perfect synchronicity with our spellcasting. As long as the spell is CAST while the grounding totem is still alive, it'll be redirected.
Sometimes they get lucky and eat your 4x EM-CL. One shaman surviving your I WIN button... hmph.
We use it to our benefit anyway. If you're doing a pull like in the last boss of Blood Furnace, you can get like 5 of those guys' Shadowbolts to all hit one totem at a time depending on how you pull and gather them up.
So turn your weakness into your strength!