erc-auto-query

erc-auto-query is a variable defined in `erc.el'.
Its value is

window-noselect

Documentation:
If non-nil, create a query buffer each time you receive a private message.
If the buffer doesn't already exist, it is created.

This can be set to a symbol, to control how the new query window
should appear. The default behavior is to display the buffer in
a new window, but not to select it. See the documentation for
`erc-join-buffer' for a description of the available choices.

You can customize this variable.