I can see two possibilities for that. The first is that Kyrie over there's right and Sigrun killed herself to save our bacon. Second's... well, we've known roughly what the timer is for a while, but what if someone wanted to test the precise length and method of it? They had her ready to be killed at any moment?

[ Admittedly, there's 0 evidence for the later, so it's just speculation. Wild speculation. But it's something she thought of by trying to think laterally??? ]