Sometimes breakers get weak, and won't hold their rated load; some brands are worse for this. I had this problem with the kitchen circuit that my MW oven was connected to. It would also do it with the toaster or waffle iron. No problems if using mixer or blender, just high wattage items. The breaker was FPE, known for poor quality. That panel has been removed. Another panel is Cutler-Hammer CH style, and no problems with it. All the new panels in the house are Eaton CH type, which I believe to be the best made. Square D type QO is also good.