Main breaker and branch breaker tripped simultaneously
Very simple: Massive power overload from the oven.
The 70A and 50A tripping at the same time means:

Problem is past the 50A. Since that has just one appliance, we know the source of the overload.
Since 70A tripped, must be greater than 70A, which of course is also greater than 50A.
Since breakers are generally designed with a thermal trip curve to avoid trips from motor startup and similar short surges, this was likely far more than 70A, because if it was 71A it would trip the 50A quickly but not the 70A.

My guess is an element (oven or cooktop) failed but short circuited along the way, tripping the breakers.
I'd be extremely wary of turning on the 50A (the 70A is fine) without getting the oven checked by a professional first. If you're in a rental, call the landlord.