The attacker should roll a Stealth check with advantage.
The burrowing monster is hiding (under the ground) and heavily obscured (because the ground is in the way). Perception checks to detect a heavily obscured creature have disadvantage.. You can either let the burrower roll Stealth with advantage or give the victim -5 on their passive Perception to ...
You can roll a stealth check against their passive Perception -- but don't have to. You, the DM, decide when a character is surprised:
The DM determines who might be surprised.
Yes, your monsters would qualify as unseen attackers if they are hidden (should require a Stealth check), and gain advantage on the first strike.
When a creature can't see you, ...
By RAW, the rogue can sneak attack as long as the target has an enemy within 5 ft of it
From the Rogue's Sneak attack:
You don’t need advantage on the attack roll if another enemy of the target is within 5 feet of it, that enemy isn’t incapacitated, and you don’t have disadvantage on the attack roll.
and from the Basic Rules(Emphasis Mine)