The phrase "flies open" typically implies something unexpected or sudden, and can have negative connotations. There are numerous synonyms that can be used to convey similar meanings, including "bursts open," "ruptures," "explodes," "pops," "splits," and "tears apart." Each of these words can be used to describe a situation where something unexpectedly and rapidly breaks open or comes apart. Similarly, the phrase can be replaced by a range of related expressions such as "unexpectedly comes apart" or "suddenly gives way." These synonyms can be particularly useful when seeking to convey a particular mood or tone in written or spoken communication.