there are 2 possible reasons for this as far as I can tell, one is not having an instant flask in the instant flask slot (5 I believe) the other one is that you havent regged above your activating threshold, which is quite logical, flaskmanager scans your life, sees that its lower than what you want it to be and then activates another flask.
So you should get an instant flask, or you can also increase your life flask delay in the flask manager settings ingame