Stay bonuses [1] are probably the most common case. It's actually pretty common in a merger, especially one with a long period between announcement and integration, to hand out parachutes of various metals to key employees.
If you're the CEO closing a deal where the buying party is overpaying by $15B, they really really don't want you to quit in the middle.
If you're a rock star category manager, they still want you to stick around because you might get the job after in the combined entity, and even if you won't, who will they be able to hire to manage the category while they're doing the deal?
