It's the "brown fat".
Infants can't shiver yet, so they have the extra layer of "brown fat" on top of their normal fat, mostly centered on their upper back. It has that LOVELY name for obvious reasons, but it gets that color because it's highly vascularized. You only find that layer on arctic and ocean mammals and human infants. Keeps 'em ALL warm.
'Course, that means that in 'normal' temps... they sweat a lot to cool down. It's a LOT easier for babies to sweat to cool off. So they do it. A lot.
As soon as you notice your baby start to shiver, the sweating will decrease as well... because the brown fat starts to dissipate as their muscles become capable of shivering.