It depends on the bulb, but usually these non-dimming bulbs will have
a "capacitive dropper" circuit: the input of the bulb has a series
capacitor, followed by a bridge rectifier, and the LEDs in the bulb
connected in series on the DC output of the rectifier.
The result is that the LED current is determined by the derivative
of the current. This works fine on a sinewave, but the output of a
dimmer has steep edges so the current peaks will be much higher at that
point and the LEDs will burn.