It could be a brake issue, but it could also be a suspension/alignment issue. When traveling at a high speed and getting heavy on the brakes, you're going to be throwing a good amount of weight up onto the nose of the car. That weight getting tossed onto the front suspension can exacerbate a suspension problem, even though you may think it's a brake issue. When you're tooling around at low speeds and easy on the brakes, you're not getting that big weight shift, so it may not manifest itself.
Also keep in mind that sometimes highways have a big crown, and/or the part of the lane where your wheels travel have been run-in heavily by trucks, etc. If you cross that crown the car may do some weird things. I run steamroller tires in my Corvette track car, coupled with more aggressive alignment settings. On the highway it is very prone to pulling me around when shifting lanes and hitting those crowns and dips.
In summary, I would look more broadly than just brakes. I would say that in the majority of the instances when our customers bring these situations to us, it turns out to be something other than the brakes (although we brake guys always get blamed
).