I'm a displaced Economics professor, not a corporate finance type, but, yes, there's a well known explanation: the "Efficiency Wage Hypothesis".
This hypothesis suggests that even in a fully competitive market, it could make sense to pay employees above the market wage, as this would
a) essentially give first pick of the better workers to that company
b) increase the cost to an employee of getting fired for "shirking", and improve output by the increased effort,
and, possibly most important at the moment,
c) reduce turnover and the training costs involved.
Unemployment has certainly dropped significantly, and by more than the "official" rate. (the regular rate doesn't make a lot of sense, anyway, if structural changes are happening, as it ignores those who have given up. Use U-4 or U-6 instead).
At the same time, we are seeing "normal" economic growth of 3% for the first time in a decade.
Put these together, and it means more workers being hired--including hiring them away. So a bonus, especially if repeated, makes staying put instead of jumping jobs make sense--and the bonus costs the company less than training new workers.
Adding fuel to this fire, the reduced tax rate is a reduced cost of doing business, meaning that the optimal level of production increases for a firm--meaning even more hiring/labor demand.
Whether firms like it or not, all of these factors mean the cost of labor *will* go up, and those that don't pay higher wages are going to lose workers (and therefore production).
Bonuses and raises *before* losing workers makes sense, and cost less than finding and training new workers.
It doesn't take any benevolent motive or political alignment to explain the firm behavior, just the old profit motive (aka "greed")