The question is being asked at the Reddit forum. bitcointalk.org is not a bad place to start research, either.
Personally, I think it is fairly much a waste of time trying to get this question answered, because we already know that Armstrong is the biggest liar on earth and a fraud as well, meaning he has zero credibility.
But still, with minimal effort, we can try to come up with answers, because others have asked this question before.
...
As for 1987, 1989 and other ecm predictions

, let’s not fall for that bs, ok? He did not predict any of those events. He just stated those dates based on 8.6 years without any specifics. And if something did happen on or around those dates he came out and claimed that he predicted precisely that event. For example, MA did not predict the crash of 1987. The Dow bottomed on Oct20 which is accidently happened on 1987.8. The decline itself started earlier on Aug25. And only after the crash MA jumped in and claimed his “forecast”. Another interesting thing is that his model shows that it should have been the peak, not the bottom.
Further to that, here
Historical Turning Points Economic Confidence Model 6000BC – 2072AD he put thousands of points so naturally something happened on or around some dates. But it is clear that in the absolute majority of cases nothing happened at all, just like in 2015.75, which MA had been preaching for years as Big Bang. But let me remind you, he predicted
the US government shutdown, government debt collapse and so much else. Of course, nothing of that happened. In fact, nothing happened at all. So in the absence of anything significant, he, in order to cover his ass, used the fact that Putin sent a couple of planes to Syria as the major Big Bang event. What a pathetic charlatan!
Also, MA says that every 51-year wave is followed by deflation

, i.e. after 1929, 1981, 2033, etc. The reality is that deflation started after 1921, briefly paused and then restarted after 1926. There was no deflation after 1981 - there was high inflation instead. And now we are almost in a deflation mode but his deeply flawed model says we should get deflation no earlier than 2033. And if we go back earlier than 1929 we’ll easily find even more flaws in all his bs theory.
And just for those who are still delusional and cannot figure out what is what by themselves, check
INTERNATIONAL BUSINESS; Japanese Regulators Get a 2d 'Scalp' Under Their Belts what NYT had to say about MA back then. “a show-boating investment manager who was well known among Japanese corporate investors for his persuasive sales pitches.” Notice, they don’t call him a great programmer, forecaster, guru, economist, financier, trader, hedge fund manager, historian, scientist or anything else but a show-boating salesman. Because that’s what he was and has always been ever since the times he could not get into college. A salesman that pitches bs, for you can't teach an old dog new tricks. And every sensible man now knows it.