Since as far as the people knew, the Emperor's bloodline was always able to father children.
Arabella had a lot of siblings though so there was clearly no problem with her family line.
Thus, the people concluded that it would either be because Arabella had health problems or she was not compatible with Ferdinand.
Since as far as the people knew, Ferdinand was consecutively away, they thought that perhaps, one reason was also because Arabella already lost his favor.
The rumors continue to grow wild from there with all sorts of conclusions the people were going to.
Arabella was not worried though since she knew the truth.
However, her maids were getting concerned. They were worried that someday, she might end up feeling depressed or overwhelmed due to the pressure from the people.