Any reasons why my 18650s go very hot when I charge them?
2.5A is too high, charge current is 1925mA with 4.2V.  
A charger should stop and alarm for high temperatures, does yours?  


When batteries age the internal resistance increases, which will result in more heat when charging/discharging.
Going over the specification of the manufacturer is risky, especially when you do not keep track of the battery age with a universal charger.