When vinyl siding bubbles or oil cans there are only a few things that can be going on here.
1) If you nail the panel too tight it would not allow for expansion and contraction and the panel would bubble or bulge. But the odds are it would be all over the house.
2) If there is a window or door cut in this area and you have not left enough room for expansion and contraction, that would make the panel bubble or bulge. (My bet!)
3) You might have something that sticks out that the siding is rubbing against.
4) I have seen a BBQ grill look the same way. I once went to a house where the "siding was bad" next to a deck. I noticed th BBQ was on the other side of the deck. When I rolled it over to where the damage was, it was a perfect match!
Lastly, the product might be bad (oil caning). This is when the oil in the panel, (a petrolium based product,)is not stabilized and forms round circles in the panel. Again it should be in other areas as well.