Why does my bread crack open on the side?

The bread I made tastes better and better these days. Practice makes perfect, as they say. Well, at least taste-wise it is perfect. So now I start to work on the cosmetic side--why does my bread often crack open on the side (shown in the bottom part of the picture)?

This website gave three possible reasons:

• Your bread may have rised too fast because of too much yeast or too high of rising temperature.
• You may have added too much flour. Check the flour to liquid ratio.
• The dough may have dried out during the rising process.

In my case the third reason is the most likely. I did leave it out there fairly long (2~3 hours?) but I didn't think that was illegal. Will have to do more experiments in the future--I need more data points.