Most breakers do not trip immediately unless the overload is significantly over their rating. IOW, it can take some time. Those times it trips may be when you're using a higher setting in the toaster oven, and it needs to be heating longer. The things do wear out, and it happens more often if they are used near their peak rating than say one used to light a lamp.
FWIW, raising the voltage tends to LOWER the amps, not raise it on most heating loads. On a heating element, as it gets hotter, it's resistance goes up, so it is somewhat self limiting on the amount of current it will draw, thus limiting the current that will pass through it unless the voltage gets way out of spec. This is one reason why brownouts can be worse than higher voltages as the current needs to go up to provide the same power.