What precisely do we consider as fork? It seems that every BIP can be seen as a fork, at least a soft fork since a lot of BIPs indeed are backward compatible. How big or impactful something has to be to be considered as fork?
Not every BIP is a fork (change in consensus rules). Some are standards for wallet software (like BIP32 and BIP39), the peer-to-peer protocol (BIP152) and other areas. The BIP repository classifies a layer for each BIP, and those that change consensus rules are explicitly labeled "Consensus".
It doesn't matter how "impactful" a BIP is when considering whether it is a fork, it is quite clearly defined. If it's a change in consensus rules that causes some currently invalid blocks to be valid, it is a hard fork. If the change causes some currently valid blocks to be invalid, it is a soft fork.
Every change in consensus rules, no matter how small, risks splitting the network into two once a block is created that is valid according to some nodes but not others.