I dont know why that happens, but how. I have a reboot app inside my 3ds folder which restarts my 3DS from HB launcher. And for some strange reason, if I try to go into the HBL launcher again, I get into the Download Play screen.Mine goes to Download Play screen which i can't exit using HOME button.
So, to workaround that, I had to power my 3DS down and up again using the power button. Then my HB launcher works again.
To exit the Download Play screen just hold the power button down until your 3DS shuts down. If it goes into the Download Play screen every time you try to go into the homebrew launcher, I sadly cant help you. :&