Form 990-T is for Unrelated Business Taxable Income, and Fidelity happens to have a nice help page about the form explains exactly why.
From their page, this is probably what's generating it:
UBTI tends to be generated by the following types of investments:
Limited partnerships (LPs)—businesses owned by more than one person, with limited liability to the owners for business debt
Master limited partnerships (MLPs)—a type of LP that is publicly traded and often found in the energy sector
Fidelity in this case is filling out the form on your behalf, and paying the tax from your IRA (or your family member's IRA, in this case), so it should mostly be painless on your side.
If you don't fully understand what to do at this point, follow up with a tax professional.