Well I've read a lot of pages here so I'm not sure if this has been mentioned, but the OP automatically assumes that it was griefing, for which it implies there was intent. If someone accidentally griefs you it's called a mistake. I.e. they didn't do it on purpose.
By using DDoor you open up yourself to not just griefers (which I strongly feel there are very few) and the more likely option of people doing it by accident. I mean as much as I hate waiting for DDoorers (Is that even a word? It is now!!!) I would never click the altar. However I don't keep a constant vigil of who has cast what and when. Also you get some who cast and Chatty Kathys typing away about how they got the kill while they should be exiting.
It has been said many times in here, but seriously recalling or death is the fastest ticket out. Even if you wait for Harry to be completely dead and click it right away you will most likely make it out. If you are even the slightest bit worried at 5% step away from the party and leave. By stepping back you won't get hit/interrupted and can get out. At 5% unless it is a terrible run and healers are at 0 SP somehow no one will mind. To be honest I usually wait until he just dies and recall if I am repeating shroud (which I never do any more since I have seen the light that completing is actually more effective especially with multiple characters). In all of my times recalling I have never once had the alter hit before I got out.
So you asked for how to avoid grief and there it is. Don't invite it either accidentally or intentionally by recalling. Also as a little tip: Wait to say "Thanks for the run guys. Take care. It was nice running with you all." until AFTER you are out. I've seen people "griefed" that way as they typed instead of left.