viewWillAppear vs viewDidAppear memory error - ios

Question

In my game, I have a function that builds the world map and configures associated data structures. I've been calling it from the the associated View Controllers' viewWillAppear function for months, and everything works as intended.
Today, I got the bright idea to move the world setup call to viewDidAppear and then display progress indicators as it does its thing. At that point, the setup function began throwing exceptions about 98.5% of the time. The few times it actually worked, I received memory warnings. Moving the call back to viewWillAppear solved the problem (and warnings) completely. But I couldn't let it go that easily and continued to debug.
I have this segment that decides where to build rivers. In that segment, there's a check for the altitude of neighboring cells to ensure rivers always flow downhill. In that segment I simply used bubble sort of the four (non-corner) neighbors, which is called while evaluating possible river locations:
for (int i=0; i<4; i++)
{
for (int j=0; j<4-i; j++)
{
if (candidates[j].tileType > candidates[j+1].tileType)
{
struct riverCandidate temp = candidates[j+1];
candidates[j+1] = candidates[j];
candidates[j] = temp;
}
}
}
Eventually, I got far enough to NSLog the values of the candidate[ ]'s before and after the sort. What I found is that when this whole thing is called from viewWillAppear, it works flawlessly 100% of the time. However, when called from viewDidAppear, occasionally one of the candidate[ ] elements is randomly corrupted (resulting in the aforementioned exception when I subsequently try to access it). I can't say I detected any clear patterns of corruption except that a) only one element of the 4 were ever corrupted at a time and b) I only saw the first [0] and last [3] elements corrupted. This chunk of code may be called a few dozen times in the course of world setup, and most passes are successful... but it only takes one of those passes to fail to screw up the works downstream.
To fix it, all I did was move the definition of temp outside the loop, like so:
struct riverCandidate temp;
for (int i=0; i<4; i++)
{
for (int j=0; j<4-i; j++)
{
if (candidates[j].tileType > candidates[j+1].tileType)
{
temp = candidates[j+1];
candidates[j+1] = candidates[j];
candidates[j] = temp;
}
}
}
Now it's working 100% of the time when called from viewDidAppear, and no men warnings.
So here are my questions: why would behavior change between viewWillAppear and viewDidAppear? If there's a potential race condition with malloc'ing the temp structure, why would it never happen from viewWillAppear and virtually always happen in viewDidAppear?
I'd like to understand this behavior better so that I don't stumble into it again.
UPDATE: So moving the definition of temp didn't actually help. Guess I just got lucky with a series of successful runs that made it appear things were working. After more debugging, I finally noticed the j loop above has a typo that lets it write beyond the end of the array, and corrected that.
But that still leaves me confused: how did I never hit that when called from viewWillAppear, but always hit it when I moved the function call to viewDidAppear? I must be missing something obvious.
Thanks