Probably low RPM's and the right combination of things kicking on at the same time causing a high amperage draw. Not really a huge deal while you are driving unless you do a lot of low speed in city driving.

 

...usually in the summer when the fans kick on.

Not a big deal and certainly not something that is going to cause me to remove the pulleys. And it isn't like they go out, they just dim a bit. My point of bringing it up is that some folks may also see this happen and be troubled by it. Since I know my electrical is in fairly good condition (new alt., new battery, Iceshark full boat everything) and I know that this does not happen when I the underdrive pulleys are not installed it either has to do with the alt. being underdriven, the fans being old and drawing too many amp or both.

 

Yeah, what he said.