On my vanilla server I play with my friends, it seems occasionally creepers do not wait to detonate at all and will explode the moment they are in range! This happens on my server. I am not sure if the same happens in Single Player but I assume it does. It is extremely annoying when I am mining because I have no chance to escape and will immediately die.
The only mod we are running client-side is optifine, which I don't think has anything to do with it, but just in case.
Hope this can be resolved!