Oven tripping circuit breaker
It is really hard to tell just what may have happened from the very brief description but clearly some short or over current situation has happened. Here are some possibilities to consider:


The replacement heating element may be the wrong one. An element that is designed for operation at a voltage of say 120 VAC would draw around twice as much current if it were connected to 240 VAC and as you can see that could trip the breaker.
In the replacement process the wires that feed power to the element connection point may have been flexed in a way that the insulation has cracked and the wires shorted together. A direct short will lead to a breaker trip too.
And finally this case, which is actually most likely, that during replacement of the element a wire that feeds power to one end of the element has had its insulation compromised or a bare part of the electrical connection has been allowed to touch the metal frame structure of the oven. Thus creates a short between the hot lead of the element and the GND of the oven structure and would cause the breaker to trip.


In any case this is a pretty dangerous situation and you should not try repeated activation of the circuit breaker to see if the problem still persists. The oven should be fully unplugged from its outlet and appropriate inspection and trouble shooting needs to be done to see where the short circuit or overload is coming from. If you do not have the tools and/or experience to proceed then it may be advisable to call a professional appliance repair technician in to help.