Have not seen a soft enrage. You can see the hard enrage b/c the boss gets a buff..
The last boss of HM BT (Yadira) will ALWAYS get the enrage buff at 10% like clockwork. You can DPS her down in 2 minutes, you can do it in 20 seconds, she'll always enrage and start killing people. It's meant to force people to pop cooldowns and kill her.