Let's say that in an alternate universe, humans developed on a relatively small supercontinent, so their languages and cultures were somewhat similar. After a while, all of the nations merged into one nation with the same language. Soon afterwards, they developed interstellar travel and proliferated across a handful of nearby solar systems.
However, something happens to this empire, and the interstellar language quickly fragments and evolves into completely distinct languages. This all happens within the span of a few centuries, enough for humans to re-establish communications and re-connect the empire. However, when they do so, they discover that the other humans speak entirely different languages.
Disregarding any plot holes in the above scenario, what event might cause this?