When you heat hypergolic fuel up enough it will spontaneously ignite. It’s got its own oxidizer in it, essentially.

Kinda. The RCS is fueled by two fluids that spontaneously combust when mixed: N2H4 and N2O4, aka hydrazine and nitrogen tetroxide (or dinitrogen tetraoxide).

If the N2H4 gets hot enough, it will exothermically decompose into a large volume of hot gas. Its sometimes used as a monopropellant - squirt some of it onto a catalyst and direct the gas out a rocket nozzle, and you’ve got a simple, reliable thruster. Mixing it with N2O4 produces hotter gas, resulting in a thruster that gets about 35% higher fuel efficiency.

Problems with the helium system could make hydrazine decomposition in the fuel lines more likely to happen. One of the ways the hydrazine is kept cool while its in the fuel lines is via high flow rate, but that requires the fuel tank to be pressurized by helium - clearing the fuel lines after thruster shutdown may also require helium. Low pressure could lead to lower flow rate and possibly cause cavitation (which can cause tiny spots of very high heat), which could in turn cause the hydrazine to decompose in the fuel lines/tank. At that point, mixing with N2O4 would be overkill - Starliner would already be destroyed before the oxygen started burning.