As meat soaks in the brine, it exudes blood/fat/hormones. These can cause excess bacteria and make the curing brine thick and moldy, as well as using unclean containers, re-using brine again, (a no-no), temps above 40° and many other factors, including the sugars/molasses too. All constants in fermentation.
Check your curing brine(s) daily. As soon as you notice something wrong, simply change the curing brine and wash off the surfaces of the meats being cured.