The phrase "happening up on" can be used to describe stumbling upon or discovering something unexpectedly. The antonyms for this phrase would be planned, expected, found intentionally or deliberately. When an event or situation is planned or expected, it is not happening up on someone. Conversely, finding something intentionally means that one is actively searching for it, rather than stumbling upon it. Therefore, antonyms for "happening up on" include words like anticipated, sought-after, purposely found, and premeditated. One can use these antonyms when describing events or situations that were not discovered accidentally, but rather, were planned for.