Spirits are a symbiotic race. The bond they have with their partners is so strong that the death of one of them may result on the demise of the other.
I say may, because if done quickly, a Spirit with a violently severed link can be saved by bonding it to another person. The chances of it succeeding are oftem low, tough. But for Hope, we can see it worked out.
Why and how did that happen is something that you’ll have to wait to see in the comics!