Finally, a subject I've studied for years, both personally and from an outside perspective (thanks to all the willing and sleeping participants ).
Briefs cause bunching, and retain heat, if you sweat its like sitting in a plastic chair naked... only with skin on skin. Boxers cause less of this issue, but not during hot weather, and at least from activity things still flop around and can get turned or pointed in places you dont want them pointed. We have body sweat and humidity, as other factors as well. Oh and hair... hairy men will have this problem more. Trimming helps, but if the person isn't as naturally hairy as others, then I found deodorant helps (the gel sticks with the cooling sensation are kind of awesome down there).
tl;dr answer: shaved pubes and gel deodorant down there