how to trace unexpected fiq source?

since the processor been powered up with EL3, AARCH64,

its pc jump to vector table fiq(offset 0x200)

but no outter interrupt signal was set,

• how could I trace the interrupt source?
• is there any possible for an instruction error to cause fiq?

even though this fault could be neglected by setting DAIF[1:0] as 2'b11,

what I need is finding out the cause.

thanks.