Because I didn’t know what a fork bomb was:

a fork bomb is a denial-of-service (DoS) attack wherein a process continually replicates itself to deplete available system resources, slowing down or crashing the system due to resource starvation.

[…]

A classic example of a fork bomb is one written in Unix shell :(){ :|:& };:, possibly dating back to 1999, which can be more easily understood as

fork | fork &

} fork```

In it, a function is defined (fork()) as calling itself (fork), then piping (|) its result into itself, all in a background job (&).

The code using a colon : as the function name is not valid in a shell as defined by POSIX, which only permits alphanumeric characters and underscores in function names. However, its usage is allowed in GNU Bash as an extension.

Wikipedia