I can think of a few reasons:

- Test commences prematurely when people are still around

- Test is aborted partway through but then spontaneously resumes when people have started coming back

- Error in design or failure of hold-down structure turns static fire into dynamic fire, moving fire to where people are

These are unlikely, of course, but they are the things we have to seriously think about and try to design out of the system in order to create safe systems.