I have been banned from kf discussions, lol, but I know the answer to your question, so:

Fleshpound can rage from damage, but that's not the only way. It can also rage when you are in its sight for too long, without it getting in attack range.
To prevent that from happening you need to either break the line of sight, just hide behind something for a very brief moment, OR you can provoke him to attack, by getting close and dodging his attack. It is possible to dodge, but even if he hits you, his rage timer is resetted, so it might be worth to take that damage.