Because astronomics is involved it feels like one of those things that either happens all the time or has a 1 in 10^100 chance of happening.
Is there another plausible explanation here? Some kind of eclipse maybe.
With 10^22 stars in the observable universe distributed uniformly we would expect something like 10^9 stars to supernova this year.
But we have to consider the speed of light. So what we are really asking is something like what is the probability that light will arrive at Earth in some interval from a supernova.
I'd imagine this is some integral over spherical shells receding into the distant past where there is some function defined on the shell which tracks stellar density as well as supernova probability. Beyond my expertise to even attempt napkin math.
We really don't. (We've already largely accounted for it, by only counting the stars in the observable universe.)
So yes, stars supernova at a rate of ~30/second, from our current frame of reference. However, many of them are occluded by dust, or are too faint to differentiate from their neighbours/parent galaxies, or are too red-shifted away...
"We note that the chance probability of this discovery is of the order of 10^−6 assuming a duration of 1 h and one supernova per century per galaxy. If we consider other factors, such as the sky conditions of the observing site and the location of the supernova away from bright host-galaxy regions, then this probability decreases by one order of magnitude."
If between 10^6 and 10^7 cameras have been tried out by taking pictures of space, then this one supernova incident is right at about the expected value.
That's... astonishing.
Even our Sun doesn't go Supernova, it just blows up a bit and discards it's outer layers, forming a white dwarf.
The stars that go Supernova are much more massive (around 3 times, IIRC) and have orders of magnitude shorter lifetimes (measured in only a few hundred million years at best).