The antonyms for the phrase "be concurrent" are "be conflicting" or "be contradictory." When two things are concurrent, they are happening at the same time and in parallel, with no interference or opposition. However, when two things are conflicting, they are in opposition or disagreement with each other, and when two things are contradictory, they are mutually exclusive and cannot exist together. Thus, the antonyms of "be concurrent" suggest a state of discord, disparity, or incompatibility. These antonyms are used to describe situations where events or actions are not aligned or are in conflict with each other.