I notice this happens when said station has a surplus of items (due to reasons) and then suddenly the stations automatically acts as provider, severely disrupting the whole network.
Why does this happen, and how do I prevent it?

Here you can see I've even removed all the constant combinators - it used to be a requesting stations that requested upto "200k items"
however it has gotten a surplus of 7800 items and then even when unmarked - or marked as requester - it acts as a provder and trains are pathed to it.