Whenever two elements are moving at the same time, through restrictive terrain, it’s very hard to have situational awareness on where that other element is. It gets even harder if the enemy opens fire on you, and then begins to maneuver themselves as well. If time permits - and it didn’t in their situation - you want to utilize movement techniques that allow one element to provide overwatch for the other, then switch (three are called overwatch, traveling overwatch, or bounding overwatch). If time is too much of a constraint, you do what you have to do, but you assume risk. Here, that’s exactly what happened. The risk that was assumed unfortunately occurred.