Originally Posted by
sethb
They might have put their bags by their own seats.
If I notice someone putting a bag overhead and walking back, I'll do whatever I feel is appropriate (often, telling a later-arriving passenger in my row "That blue bag belongs to someone seated much further back, so if you take it down he can't get here in time to object.")
If this happened in F, I would agree. But in an all-coach plane, it's first come, first served.
If OP can find a willing pax, switch seats just before landing, so no one has to swim upstream.