For some people it is, but it can require a lot of work and practice to get there. Make sure you're very aroused before starting anal play, as that helps.
Genital play also helps a lot. If you want to go for the straight anally-triggered orgasm then you can start "training" your body by first having orgasms (using genital play) while enjoying anal stimulation, then use genital play to get very close to orgasm and anal play to push you over the edge, then back off slowly. You'll need to practice to find out what moves give you the best anal play.
Lastly, always remember that it isn't a requirement. You can have plenty of fun without having an anally-triggered orgasm.