All of a sudden grub menu started showing

I have a pure Ubuntu laptop (no dual boot). Today when I restarted, suddenly grub menu started showing. After I select Ubuntu, it boots boots as normal. So my questions are:

1. Why did it start appearing all of a sudden?
2. How do I hide it?

EDIT: output of cat /etc/default/grub:

# If you change this file, run 'update-grub' afterwards to update
# /boot/grub/grub.cfg.
# For full documentation of the options in this file, see:
#   info -f grub -n 'Simple configuration'

GRUB_DEFAULT=0
GRUB_HIDDEN_TIMEOUT=0
GRUB_HIDDEN_TIMEOUT_QUIET=true
GRUB_TIMEOUT=10
GRUB_DISTRIBUTOR=`lsb_release -i -s 2> /dev/null || echo Debian`
#GRUB_CMDLINE_LINUX_DEFAULT="acpi=noirq"
#GRUB_CMDLINE_LINUX_DEFAULT="acpi=force"
GRUB_CMDLINE_LINUX_DEFAULT=""
#GRUB_CMDLINE_LINUX_DEFAULT="quiet splash acpi=force"
#GRUB_CMDLINE_LINUX="acpi=force"
GRUB_CMDLINE_LINUX=""
#GRUB_CMDLINE_LINUX="acpi=noirq"

# Uncomment to enable BadRAM filtering, modify to suit your needs
# This works with Linux (no patch required) and with any kernel that obtains
# the memory map information from GRUB (GNU Mach, kernel of FreeBSD ...)
#GRUB_BADRAM="0x01234567,0xfefefefe,0x89abcdef,0xefefefef"

# Uncomment to disable graphical terminal (grub-pc only)
#GRUB_TERMINAL=console

# The resolution used on graphical terminal
# note that you can use only modes which your graphic card supports via VBE
# you can see them in real GRUB with the command `vbeinfo'
#GRUB_GFXMODE=640x480

# Uncomment if you don't want GRUB to pass "root=UUID=xxx" parameter to Linux
#GRUB_DISABLE_LINUX_UUID=true

# Uncomment to disable generation of recovery mode menu entries
#GRUB_DISABLE_RECOVERY="true"

# Uncomment to get a beep at grub start
#GRUB_INIT_TUNE="480 440 1"