How do I get GNU screen not to start in my home directory in OS X?

A better way to check what exactly is being run when you type screen would be to run command -V screen. This will tell you whether it's running a binary (in which case the full path will be given) or a shell alias, function, etc.

Another thing to check is whether there's a chdir command in either your ~/.screenrc or the system-wide screenrc.

The screen(1) man page describes the Screen chdir command.

   chdir [directory]

   Change the current directory of screen to the specified directory or,
   if called without an argument, to your home  directory  (the  value  of  the
   environment  variable  $HOME).  All windows that are created by means of the
   "screen" command from within ".screenrc" or by means of "C-a : screen
   ..." or "C-a c" use this as their default directory.  Without a chdir command,
   this would be the directory from which screen was  invoked. …

My Screen configuration for a programming session includes the command

chdir "$HOME/Projects"

The first step is to make sure that screen isn't an alias.

Type alias and look for screen. Be warned that this list could be long. You may want to type alias | grep screen to separate the wheat from the chaff.

If screen turns out to be an alias for something like cd ~ && screen you can remove this using unalias screen.