This happened a week ago too . I can't put up a search in lfg because my group gets merged .
I don't know if this is a personor or a bug but this drives everyone mad.(because its happens to the other groups too)
I personally have never encountered this as bug. When your group merges, it means either someone in your or the other group hit the merge button.
When it was on accident or something and everybody is upset, if they speak up about it is another question.
If it keeps happening repeatedly and is actually a bug, consider writing a bug report.
"As you know, those who you once called friends have become enemies." ~Glint
this is sometimes is "evel bug". Some people thik that this is fun, and I see that tendentious more often ...
So for example 2-3 ppl is stay on ark, someone come in, and do merge with other 1/5 pary. So who is near ark get instant kick from last boss.
Why ppl not have accept window if someone merge ?
Most likely just trolling. GW2's LFG/grouping system is... special.