Good question, and frankly my answer may be a bit shaky. Hold on though.
The tricky aspect of your question (whether you realize it or not) involves two things: timing and perspective.
You seem to imply that only in looking back do you realize "decision X" (for example) was a harbinger of things to come. At the present, however, only from your perspective of hindsight, was your "decision X" a harbinger of all the events subsequent to it, including the last really bad event in that chain of events.
(You'll notice I'm assuming that each link in the chain which culminates in the really bad event is also bad, at least to some extent. Correct me if my assumption is wrong.)
At the time of the first--the "kick-off event"--in the chain of events, you did not realize the first event was a harbinger. For that reason, in a funny sort of way that first event could be described as the sine qua non, which is a Latin phrase meaning "without which, not."
Put differently, your "decision X" turned out to be the event without which the last bad thing would not have happened (i.e., the sine qua non).
I know that usually the phrase sine qua non means the absolutely necessary thing upon which everything which follows depends. For example,
"The perfect cake is the sine qua non of the carefully planned modern wedding"
That means a carefully planned wedding (and, I assume, a successful wedding, which is a good thing) cannot happen without there first being a perfect wedding cake.
Why, I ask, cannot the first bad event in a series of bad events leading up to the last really bad event be considered the sine qua non? After all, the first bad event got the ball rolling, did it not? Without that first event, the last bad event would not have transpired.
I suppose another possible word for that bad, first event could be linchpin, which means
something that holds the various elements of a complicated structure together.