To answer the original question:
– Riding on the flat didn't cause a bulge by itself.
– What kind of flat was it? Maybe you broke the casing while you got your flat. In other words, if you had repaired the flat right away, you might have discovered that the tire was ruined anyway.
– Riding on a flat tire creates a crease. You create pressure points where the rim tries to cut through the tire and a little further away where the tire is folded flat. If you ride long enough, you'll cut the wire and could end up with a zipper cut*. So if your tire had a lot of mileage, you might have accellerated something that would have happened a few months down the road anyway.
– If you ride on a few crevasses with a deflated tire, you have the risk of flattening the rim or doing a snake bike in the tube.
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* The zipper cut happens when truck tires have been run for a few hundread kilometres at very low pressure and are then repaired. They may have a weakened casing and will litterally rip apart at mid-height in the shop, soon after being repaired. The same isn't likely to happen with a bike tire, but you see the imagery.