I'm not involved in these discussions internally so take this all with a grain of salt, but I still have more insight than speculation. To be reaaallly accurate, there's technically still one budget for the team. However it does get broken up further into the specific areas depending on the goal and vision of the product, and how many resources it would take to accomplish the high-level vision.
For example, gamemodes and UI often share resources, and have a faaar greater headcount than gameplay. For whatever reasons, it takes significantly more people to make screens and gamemodes than it does gameplay. Audio is even smaller, with only a handful of people. This process is not one of cannibalization, though. If one area has bitten off more than they can chew, we don't really give them resources from another area, but either ask for more from up on high or simply cut some of the features we wanted from elsewhere in the area.
This is a bit oversimplified, but if we wanted 5 new game modes and 3 cool new gameplay mechanics but GM only had resources for 4, for instance, we wouldn't sacrifice a GP thing to keep GM afloat. They'd just prioritize their 5 and drop the lowest one.
The central art teams (modeling, characters, environments, lighting, physics rigs, etc) are more like contractors within EA itself, and each game deals with their managers etc to figure out who/what we need, and then we get their time temporarily or split with other projects. UFC's art design team would have a budget allocated towards new art content as well, and would divvy that up between new fighters, venues, updates, etc. If they think that there's something they can't get but would reeeaaally want, then generally they would make their case to the higher ups for more funds based on the return of value.
It's a very complicated process, and I do not envy one bit the people that have to try and figure out how the budget gets used, or how to convince the powers that be to give us more money when we want it. Bless their hard-working little hearts.