thief_39: Thanks guys, should have mentioned. Yes, if I start up and play for a short time (in this case around half an hour) then it will close normally. So that's what a memory leak looks like then? Huh. Never knew.
That's how it looks from a user perspective on an OS with virtual memory (basically, any modern OS that you would likely interact with).
On an embedded system without virtual memory, what would happen is that the device would malfunction at some point, as it tries to access memory that doesn't exist. Alternatively, the program could be written to detect that situation and shut down if that happens. (Prediction: If you disabled the page file on your system, Wasteland 2 would crash after a while, possibly taking other parts of the system with it.)
From a programmer perspective, a memory leak looks something like this (code is in C):
{
x = malloc(sz);
/* code that uses x */
/* no call to free(x), or the call never gets executed because of a bug */
}
In other words, the programmer forgot to free the memory that was allocated, so the system thinks the program is still using the memory and can't reuse it for anything else. Then, of course, the program allocates more memory, and then the system eventually runs out of memory and has to swap something to disk. Once data has been swapped to disk, if it is later accessed, the system has to swap it back into memory, and that's the disk access you have been seeing.
Memory issues are actually one of the trickier thing to debug in C programs, to the point that tools have been developed to try to deal with it. (One recent language, Rust, has strict rules (that can trip up programmers not used to them) that are aimed to prevent memory related bugs.) Memory leaks, in particular, can remain undetected because they don't cause the program to crash or behave oddly; they just cause the program to eat memory if allowed to run long enough. (Other errors, like memory corruption, will crash the program if you are lucky, and can lead to remote code execution vulnerabilities if you are not.)
Incidentally, my suggestion of adding more RAM would only delay the inevitable; you would be able to play for longer without the system having to swap, but it would still happen eventually.