I found out if I create a Hslider using the shortcut 'Command + Shift + h', the Pd window hides in the background. 

Other shortcuts seem to work without the problem so I guess it clashes with the Mac's OS shortcut 'Command + h' which is used to hide the windows of the front app.

I didn't have this issue on Pd-0.47-1 and I hope this can be fixed in the stable version of Pd-0.48.

Thanks,

Zack