A large number of players will at times get careless moving between objectives in Coop, expecting that they will respawn as soon as their teammates take the next objective if they die, as that is what happens most of the time. If they do end up dying, and the next objective then instead activates a ~2 minute long counter-attack, their remaining teammates will have a harder time defending themselves, and the mission may fail, spoiling everyone's fun. If the checkpoints that do trigger counter-attacks had special markers as a warning, or if the game would tell the players how many counter-attacks they have yet to encounter during a mission, players would know to not get as reckless as they often do now.
As far as I know, whether or not an objective triggers a counter attack is random. Not knowing what's going to happen when you capture an objective is part of the fun and atmosphere of the game. The only time you can predict what will happen is that there will never be a counterattack when there is only one player left alive.
Never move carelessly in a war zone.
As far as I know, whether or not an objective triggers a counter attack is random.
If it works like Ins2014 does (which is a stretch as Sandstorm changed everything from the move to Unreal) then the chance of a counterattack varies depending on the number of alive teammates. I think in Ins2014 the chance was 20% with a boosted chance if there were more than two players alive, per player.