Your ambient temperature is high enough to not have the heat on. When you turn it on it's getting too hot for them, and they can't get far enough away (unless your brooder is the size of an entire room) so they start showing signs of heat stress.My chicks are bouncing between heat lamp and no heat lamp currently. Its about 82 to 83 degrees ambient in their room. They are generally quiet, but when the temp drops, I turn on the heat lamp. I can only leave it on for a total of about 10 mins and about 3 of my chicks start panting.
Ultimately, their behavior and body language tells you if they need heat or not.