There was some changes made whilst Myth Drannor update was released, we're not exactly privy to what exactly happened, but we do know that the 32bit and 64bit clients can calculate stuff "differently". And thus, certain stuff that might be seem normal and identical, actually ends up different between 32bit and 64bit clients.
What this means is, say for example the draw distance for high resolution stuff is normally set at 99 meters and nothing beyond that point is rendered at such high resolution and detail. Now lets say the graphical team added new effects in and put them above 99 meters - say at 100 meters, but because they're not normally drawn at high resolution, it's not a problem, so the GPU doesn't fall over with the client (and take the OS with it for example as it's often quite linked and tied together these days). But lets say that 99 meters was then seen as 100 meters; this then tells the client to render normally what is only seen as distant stuff (and doesn't need high resolution) and now it's all over the skydome and there's lots of it. As you turn around, stuff gets swapped in and out of the VRAM and system memory and before long, it's full before it can be released and then it hangs not knowing what to do.
That's just one example that could happen.
But as said, we're not privy to exactly what was changed that caused this, but I can certainly see something like this being an issue.
J1NG