MBP reset itself overnight .....

Hi a very strange thing happened over night, allow me to tell you about, for a week or two now i have been using my MBP plugged into the mains all the time (in the hope that the battery will last longer overall), i went to bed last night, closed the lid and pulled the plug out, i assumed it was on stand by (gone to sleep), i got up this morning, plugged in the WiFi router and plugged in my MBP, i didn't open the lid.

The next thing i knew i heard the boot up sound from inside the closed lid of my MBP !!!, funny i thought ?.

Opened the lid the screen showed it was still booting,, in a short while the desk top screen came on there was two messages on the screen, firstly it had forgotten my Router WPA password !!!! secondly the time was reset to 1 january 2001 the time also reset.

Happily, after panicking for a while i found my password, typed it in, got connected to the WWW and my date and time quickly restored itself to the correct time.

And another thing, the battery was run down almost to nothing about an hour left !!.

Can anyone please explain what happened or what went wrong ! thanks.

Tim