You saw a silent truce that wasn't seen by the other. Your ass was kicked as the other 2 seem to have agreed to take you out. There is no obligation to take flags.

I agree that it is strange or unwise to go for 2nd on a 5-player table where 2=3. Yellow could have asked red for a roll or you all could have gone for a 3-way. But unless this happens more often, it is nothing to make a big fuzz off. Maybe they just enjoyed your anger...