Where did all the monarchs go? (again!)

It's not so long ago - September 2019, just nine months ago - that I was writing about how the populations of monarch butterflies were suddenly booming, after a severe dip the year before. Well, guess what, they're back in the doldrums again this year. We have seen a total of ONE monarch this year here in Ontario, and we are nearly into July!
I'm not saying that all is necessarily doom and gloom, and that extinction is surely nigh - we have already seen how quickly their numbers can bounce back. But what is normally our commonest sunmer butterfly is suddenly a rarity, and psychologically that's hard to deal with.
So, where did they all go to? The latest survey of the over-wintering population in central Mexico (and we were there in February, as it happens!), shows that the area of forest occupied by monarchs was down 53%, from 15 acres in 2018-19 to a measly 7 acres in 2019-20. They were certainly harder to see when we were there in February than usual, apparently, although it was still a splendid sight. This graph of monarch over-wintering populations (from the Monarch Watch website) shows how numbers have declined, but also how inconsistent they are:

The best explanation that the researchers could come up with is that temperatures in southern Texas were substantially lower than usual in March and April 2019, leading to slow growth for the migrating monarchs' eggs and larvae, and fewer monarchs in the following generations that continue the migration up to Canada. Therefore, there were fewer making the big migration back south to Mexico in the fall of 2019, and therefore fewer butterflies over-wintering in Mexico this year. That's the official story, according to the National Commission of Protected Natural Areas of Mexico and WWF Alliance-Telmex Telcel Foundation, which conduct the count each year.
EXCEPT THAT... that was definitely NOT our experience here in Ontario. Rather than a dearth of monarchs in the summer of 2019, we experienced, at least anecdotally, a bumper year - as good as, if not better than, the big bounce-back summer of 2018. So, that explanation makes no sense to us, although the poor over-wintering populations does explain why we are seeing so few here this summer.
It makes you realize that, much as we now know about the once mysterious monarch migration, there is still much we don't know, even now. The only good thing we can say is that we do know, from past experience, that those resilient little insects are capable of bouncing back from a dire situation.