The term "by surprise" can be replaced by a variety of synonyms, depending on the context and tone of the sentence. Some common alternatives include "unexpectedly," "unpredictably," "out of the blue," "by chance," "abruptly," "startlingly," "astonishingly," "suddenly," "unanticipatedly," "without warning," "upset," "taken aback," and "caught off guard." These phrases can add nuance and color to writing and speech, allowing the speaker or writer to convey a particular emotion or tone more effectively. Regardless of which synonym is chosen, the underlying meaning of the phrase remains the same: something unexpected or surprising has occurred.