The colon works as a modifier in
zsh. Quoting this reference,
After the optional word designator, you can add a sequence of one or
more of the following modifiers, each preceded by a ‘:’. These
modifiers also work on the result of filename generation and parameter
expansion, except where noted.
Turn a file name into an absolute path: prepends the current directory, if necessary (...)
All in all, the
:a works as a modifier and prepends the current directory to
VAR expanded value.
In order to prevent the modifier from acting, enclose the variable in curly braces: