Agreed. It can really be an HR nightmare if bonuses are perceived to be given out for subjective reasons.
It's two-fold in my mind -

Are bonuses expected by the employees? If so, what would disqualify someone from receiving a bonus, and can it be described so that the individual can work on improving that behavior to qualify for next round.

If bonuses aren't expected - the same logic applies, can you articulate why a bonus was rewarded to one person vs another?

If there isn't that understanding as to why a bonus was offered, or how to qualify for a bonus - it's confusing to the employee who wants to succeed but doesn't feel like they have the direction they need, or more problematic - feels like not qualifying for a bonus was personal despite meeting the requirements for the job.