Optimisation. You need to get your PC stable at 99% GPU.

Try setting windowed fullscreen on.

Set vsync on.

You get 50-60 fps on both lowest and highest setting? In that case it sounds like vsync is kicking in keeping your fps locked on the refreshrate of your screen. Without vsync your fps should show up higher on low settings then your screen-refreshrate, considering you run at around refreshrate framerate on ultimate settings. Anyway, that wasn't the issue here. Your question was about how the gpu load could increase so much...

That's caused by the new dedicated fullscreen mode. This mode will allow your gpu to be maxed out on performance and every bit of performance to be squezed out of it as compared to the old (and now optional) windowed fullscreen mode.
The windowed fullscreen mode is governed by windows, where windows will dicide that the game can only be allowed 30fps because of other applications being active are also request window redraws. This can vary heavily and may even go as bad as windows forcing the game to run only 10 or 15 fps for instance. Due to this behaviour, your GPU will basicly be just picking it's nose because it's limited to the amount of work it's allowed to do, while it can handle way more.
The old situation would, depending on your graphics card, maybe utilize somewhere between 50%-80% of your GPU performance capability, which also results in a nice and cool GPU, because it's most of the time just sitting there, waiting for work.
The new dedicated full-screen will just push it to the limits to get the highest FPS possible for you, but this does result it 100% GPU load and thus more heat.

If your system is becoming unstable due to this, make sure to check on the following:
* The temperature of your GPU to see if that is causing the card to crash, if the temperature goes too high, the GPU may crash, and so will your system. The GPU overheating though isn't an issue of the game, but rather an issue of either the GPU cooling, or the overall computer case airflow/cooling. A GPU is obviously designed to perform at 100% load for long periods of time and should therefor not overheat unless something is really off on cooling. This can be dust collecting on fans and in heatsyncs, or hot air not being able to vent out of the system fast enough, or cool air not being able to come into the system at a high enough rate.
* In case of an ATI card at least the R9 series Radeon (and probably some cards before), make sure you don't have the powerboost option (it allows control over the max power draw of your GPU) set to a (too low) negative value, setting it below 0% boost it can cause too little power to your card, making it unstable, especially at high load. This will result in random crashes or blue screens
* If you got a power-heavy GPU, also make sure that you have a powersupply that can support it. A too low wattage powersupply or an unstable powersupply (you can buy the crappy ones for like 20-30 euro, but they're very unreliable, especially under high-load), it could also cause lack of power to your GPU, which like above point can cause system lockups, instant reboots or blue screens.

If all fails, you can always force the game to use the windowed fullscreen mode to get the game to run like it used to before the patch. This should give you a lower GPU load again, but it will also sacrifice performance.

This problem annoys me. It annoys me very much. It's basically biggest problem with ESO which I encountered as soon as I ran the game first time.
Since I have a laptop, I try keeping it as cool as possible, I cap FPS at 48 and underclock the GPU. But the game still uses 99% GPU occasionally even at 48 FPS, only lowering down to 30 keeps GPU usage at 55%. I now need to change my thermal paste due to high amb temps due to high GPU temperature, but I don't have any yet, and the GPU warms up the sound card till the sound distorts and disappears.
I turned down all possible settings in the ini file, the game looked worse than Oblivion, and still I had 99% GPU usage. That's not a well optimised software at all if it looks like game from 2003 (lowest settings possible) but uses all 90 watts of 2013 graphics card, regardless of settings used (either ultra or lowest).