Science ain't sure exactly of the cause. Of everything I read, the most plausible theory is that parts of your body are literally overheating, they're too hot, cells are dying, and the body responds... by overreacting (like it does with a common cold). Blood vessels (tiny capillaries) expand to dissipate heat and they keep on expanding until they burst. Burst blood vessels cause other bodily systems to react: inflammation and anti-bodies flooding the area make it worse.
The two best ways to keep this from happening is staying cool and compression, which keeps blood vessels from bursting.
Blood pooling in the legs from lots of walking; and exercise pumping more blood around, both exacerbate the problem.
When I get it, it's just tiny red dots. When my friend gets it, his body has complete systemic reaction with swelling everywhere.
This reminds me... I'm determined to find cooler, more breathable sneakers...
Anyway, when blood vessels burst, it takes time to heal. Anti-inflammatory agents such as aspirin, ibuprofen, and anti-histamines can help with symptoms.