Miners can certainly initiate their own Drivechain soft fork. The question is whether they can maintain it.

The instant that a majority of hash is not defending the escrow, it’ll drain to the defecting miners. In effect, the sidechain escrow acts as a bounty, begging miners to drop the fork and take it.

This is why Drivechain truly needs the support of node operators. Nodes must be willing to invalidate the escrow piracy attempts, otherwise miners are incentivized to defect.