Why is my breaker tripping

The most common reason for a breaker to trip is excessive current. If the breaker trips instantly when the dryer is turned on then you have a short circuit, probably a short to ground. If the dryer runs a bit before tripping the breaker there is an overload. It helps to have a fundamental understanding of electricity and circuits. If you open the dryer there should be an electrical schematic to follow. I suspect you have incorrectly wired the components you replaced or you used the wrong components. It's difficult to be more specific with the information provided. Also, you may have pinched a wire when closing up the dryer enclosure.