Some detailed maps and many mods demand more than the default 8 Meg Memory Heap. To run those maps you will need to add a command line variable to the command line to increase this to .... say 16 Megs (havent run into anything needing more than 16).

For your example above you might try launching like this :
sdlquakepxd -mem 16 +map dragon

Or you could modify (as root) the sdlquakepxd script in /usr/bin to add that command switch automatically.