I don't have a PS3, but from the YDL 5.0.2 manual:
Returning to the GameOS
Your PS3 will now boot to YDL each time it powers on unless you command
kboot (the bootloader) to again boot the GameOS (see BOOTING GAMEOS,
To return to the GameOS, at the kboot prompt, enter:
The PLAYSTATION®3 may be forced to return to the GameOS by holding the
power button for about 5 seconds at power on. Doing this resets your PS3 to
factory default. To return to Yellow Dog Linux, you will need to perform steps
6 and 7, above.
and 6 & 7 above are:
6. You must now select "Other OS" in order to install YDL:
Settings ==> System Settings ==> Default System ==> Other OS
7. When prompted to boot Other OS, select "Yes".
My guess here is since you already have YDL installed it is not looking to re-install it, just boot from what is already there.