Block space is always a little bit of a bidding war, even with zero fees. That's because each additional byte added to a block increases the time it takes to propagate the block by just a little bit.
When miners find a block, they race to get it published as quickly as possible to as many nodes as possible. If A finds a block, and stuffs it full of transactions; and shortly thereafter B finds a block, and includes no transactions, B might actually "win" the race because B can propagate his tiny block much faster than A. So each txn added incurs a tiny penalty just by virtue of adding to the payload.
> they decide which fork to run
In reality most hashpower is pooled and pools autoswitch between forks depending on which is more popular in the moment. So most people running miners are mining all forks.
When miners find a block, they race to get it published as quickly as possible to as many nodes as possible. If A finds a block, and stuffs it full of transactions; and shortly thereafter B finds a block, and includes no transactions, B might actually "win" the race because B can propagate his tiny block much faster than A. So each txn added incurs a tiny penalty just by virtue of adding to the payload.
> they decide which fork to run
In reality most hashpower is pooled and pools autoswitch between forks depending on which is more popular in the moment. So most people running miners are mining all forks.