This is the reason, for any club you have to be alert and sharp until the last second because you don't know when your opponent really has the chance and the right moment to turn things around. If they already feel like they have won because of a sufficient goal difference in the last minutes and then it seems like they have reduced the strength of their squad or have thought they have won, this is what will happen. They may be shocked by the protests, but here, Dortmund should have learned more from their next mistake and anticipated counter-attacks, especially in the last minutes.