I've started to notice a recurring pattern to our team's burndown charts, which I call a "bowstring" pattern. The ideal line is the "string" and the actual line starts out relatively flat, then curves down to meet the target like a bow.

My theory on why they look like this is that toward the beginning of the story, we are doing a lot of debugging or exploratory work that is difficult to estimate remaining work for. Sometimes it even goes up a little as we discover a task is more difficult once we get into it. Then we get into implementation and test which is more predictable, hence the curving down graph. Note I'm not talking about a big scale like BDUF, just the natural short-term constraint that you have to find the bug before you can fix it, coupled with the fact that stories are most likely to start toward the beginning of a two-week iteration.

Is this a common occurrence among scrum teams? Do people see it as a problem? If so, what is the root cause and some techniques to deal with it?