Is this purely a coincidence or is there some sort of geological explanation for it? North America, South America, Africa, Asia, all have this general shape. Even Australia to a smaller extent has a narrowing southeastern area.
Almost certainly coincidence. If you can find a reason I’m willing to listen. A sample size of 6 (I’m assuming we couldn’t make a statement about Antarctica which straddles the southermost position) makes it fairly difficult to assert a non-random hypothesis.
TLDR: no reason
The fundamental unit that makes up the continental land masses is called a craton. Basically a craton is a chunk of crust that is small enough to resist being broken up by tectonic forces, so it has existed since continental crust first formed. The continents we see today are not fundamental units, but instead are made up of lots of cratons that have aggregated. For example Africa is made up of five cratons of essentially random shapes and sizes.
The continents we see today originated when the supercontinent Pangea broke up. The breaks happened at the edges of cratons, but exactly where the breaks happened was effectively random. It would have been impossible for a Paleozoic geologist to predict exactly which cratons were going to end up in which continent.
The point of all this is that the shapes of the continents are random accidents related to exactly how the tectonic plates were moving when Pangea broke up. Even if it’s true that all continents are wider at the top then the bottom (which is debatable) this is by chance and there is no deep underlying significance to the observation.