I don't have a lot of data but the electrolytics on an older riser board seem to have a slight dome, especially the one closest to the regulator (?) chip. The same capacitor on a new riser is flat or even has a slight dimple in the middle of the top surface.
In the picture below an old (+1 yr) failed riser is on the left and the new unused riser is on the right.
http://imgur.com/rcHaA1mThe difference was noticed by running a metal straight edge across the top surface of the capacitor.
The capacitor is a small pressure vessel and aluminum is a soft metal, so it's possible heating could do this.