In general, a "death spiral" is a situation where an initial setback in a battle makes losing more and more likely. Any system where being hit imposes a penalty on you has this characteristic, and makes landing the first blow extremely important.
I'm very familiar with 2nd edition of Warhammer frpg, having run several campaigns with it. I'm not as familiar with the 4th edition, but from my understanding of it, it has this feature because of the "momentum" design.
Say our hero is fighting a brigand, and they are both equally skilled with the blade. One attacks the other, the other tries to parry. In the old system, this could result in a battle with lots of misses and parries, because to land a hit, the hero has to make their weapon skill check to hit, and the brigand has to fail their parry.
In the new system, the "winner" gets momentum, so even a "miss" by the attacker (the parry was successful) is impactful because the person who parries now has a +10% momentum bonus. This is a good thing... but it sets up a death spiral. The very first blade exchange is very determinant on who will win.
So - let us say that it takes 3 hit to win this fight. On the first exchange, each person has a 50% chance of winning, because their skill level is equal. I'm going to ignore ties because 1% chance and the math is easier. Let us say that the brigand wins the exchange and now has 10% momentum. The brigand now has 59.5% chance of wining the next exchange (and land a hit). If they land a hit, not only did they inflict damage on the hero, they gained more momentum!
The hero attacks again, but the brigand now has 69.5% chance of winning the exchange. The brigand parries. No damage, but they have more momentum. Their attack now has 79.5% chance of landing!
Now, of course at any moment this sequence can be interrupted. But the odds are not in the favor of the person on the loosing side, at all, even though they are an equal match to their opponent!