I'm presuming this is happening because you have required profile fields on a user profile which don't appear on signup?
It happened to my organisation when we added a new profile after existing users already had an account, forcing an update of the new profile (as it's now a required field) when they next logged in.
Anyway, the real issue is that after updating the user profile from the page "(moodlesite)/user/edit.php" you are forced back to "(moodlesite)/user/preferences.php". This seems to be predetermined by Moodle core php and needs this to be edited in core code.
You need to look at moodle/user/edit.php around line 192 (this may change depending on the version of moodle you are using) and change "$returnurl = new moodle_url('/user/preferences.php', array('userid' => $user->id));" to "$returnurl = new moodle_url('/?redirect=1');" This will take them back to the default landing page you have set.
I recommend commenting out the old line (with "//") and adding the new one underneath so you can go back easily if needed.