[cosmie]

There's a third as well:

The loss of them is perceived to open the person doling out the raises to significant risk in the future, and they're in danger of leaving.

Generally happens when someone is unexpectedly found to be in a mission critical role with little to no redundancy or contingency plans. Shortly after your big raise the company will start working in that redundancy and contingency planning, and once it's in place you risk being seen as an overpaid resource. Making it a somewhat risky avenue to get a big raise.