antlr-indent-at-bol-alist is a variable defined in `antlr-mode.el'.
Its value is

((java-mode . "\\(package\\|import\\)\\>") (c++-mode . "#\\(assert\\|cpu\\|define\\|endif\\|el\\(if\\|se\\)\\|i\\(dent\\|f\\(def\\|ndef\\)?\\|mport\\|nclude\\(_next\\)?\\)\\|line\\|machine\\|pragma\\|system\\|un\\(assert\\|def\\)\\|warning\\)\\>"))

Alist of regexps matching lines are indented at column 0.
Each element in this list looks like (MODE . REGEXP) where MODE is a
function and REGEXP is a regular expression.

If `antlr-language' equals to a MODE, the line starting at the first
non-whitespace is matched by the corresponding REGEXP, and the line is
part of a header action, indent the line at column 0 instead according
to the normal rules of `antlr-indent-line'.

You can customize this variable.