allout-auto-activation is a variable defined in `allout.el'.
Its value is

Configure allout outline mode auto-activation.

Control whether and how allout outline mode is automatically
activated when files are visited with non-nil buffer-specific
file variable `allout-layout'.

When allout-auto-activation is "On" (t), allout mode is
activated in buffers with non-nil `allout-layout', and the
specified layout is applied.

With value "ask", auto-mode-activation is enabled, and endorsement for
performing auto-layout is asked of the user each time.

With value "activate", only auto-mode-activation is enabled.
Auto-layout is not.

With value nil, inhibit any automatic allout-mode activation.

You can customize this variable.