Hi Taitea,
Sometimes you'll get discus to go crazy and attack the food and others times they just eat slowly.
In my personal experience I've found that they only attack and devour when they are in a larger group or a VERY hungry group. If I'm away for an entire day, for instance, and they don't get fed for 12 hours, THEN they attack. Thought mine only attack for a short period of time and then back off and wait for me to drop the cube so that they can eat slowly.
Most of the time, mine are never that hungry since I feed often, so mine are much more like yours. They will come up and get a couple of bites from my hand, then wait and eat off the bottom or out of their feeder cone.
It's not a problem so long as yours are active, healthy and growing. =)