Could it be because of the weather in general? Adults are known to sweat when they sleep in the heat and humidity, I can only imagine how bad it will be for baby. However, if baby is sleeping in air conditioning and sweating continues, it could indicate an underlying condition (rarely though). Babies with congenital heart problems will have trouble eating and begin to sweat as they work hard to eat. Breathing problems, genetic disorders or thyroid problems could also cause your baby to sweat excessively throughout the day. If indeed that is the case, your babies will show other signs of illness, too, such as pale skin or slow weight gain. If you are worried, discuss your concerns with baby's doctor. My personal opinion, it's probably due to the erratic weather.
Do you constantly have them swaddled, covered up? Do they also pespire in AC? My LO also pespires - but less when in aircon room