in a traditional site to site tunnel, either side can negotiate the tunnel if traffic that matches the crypto access lists necessitates it to. if you configure an IOS device as an easy vpn server, and the pix the client, then the pix would be in the position to initiate the tunnel. only pix 501 and 506's can be easy vpn clients though.
there are some ways to create tunnels from behind a pix. a site to site tunnel, or a end user client tunnel?