If having a burrow speed was meant to cause an auto-surprise, I think that would have been accounted for in the CR'ing rules. Only mode of movement that affects CR is a ranged attacker which has flight before CR 10.
If the DM feels a burrower happens to be in an advantageous situation, there is always the advantage / disadvantage system, but it should require more than having a burrow speed. Being able to get close before combat can even start is a solid boon. Adding in also getting a surprise round of clawing and chomping is a bit much.