In U.S. law there are generally speaking, two types of bonuses.
Non-Discretionary - A.K.A any bonus that doesn't take into account discretion on part of management and higher. This is usually for bonuses that apply as an "incentive" and have requirements to achieve. Think sales targets for sales teams, on-call incentive structures, and more. This type of bonus is actually considered part of your wage.
Discretionary - A.K.A. any bonus that is paid at the discretion of company ownership. Notably these are bonuses that are not typically communicated in advance, and thus an employee wouldn't know to expect them. They might still expect them from "tradition", but if the only time you ever know about a holiday bonus is when it arrives, it's likely Discretionary. These bonuses aren't guaranteed by anyone - and an employer can indeed to choose not to pay these types of bonuses.
It seems that Twitter failed to pay a non-discretionary bonus and there's a large paper trail of incentives given to employees for this bonus. I really hope the DOL makes an example of them on this case.