Re: Why . . .
The stuff is perfectly storeable (unlike most fuels typically used in rockets themselves), so no harm in this regard. It's also not explosive at all, as long as it does not come into contact with each other, which I assume the engineers went to great lengths to make sure won't happen.
As for why it is done so much in advance, a satellite is typically given its last checkup and sanitization (there is a policy to protect space bodies from contamination with Earth bacteria, especially restrictive in case of a place where they could possibly survive, such as Mars), and encased in the fairing (along with filling said fairing with something neutral, often helium) long before it is launched. The main reason I think is that considering how finicky the whole "launching stuff into orbit on top of a giant can of explosives" stuff is, it's nice to have variable less to worry about on the final stretch of preparations.