I got home today after work to a complete 7+ hours print on my printrbot simple metal. The print is great except the heated bed is really hot. I went to the octoprint page and found the heated bed at 100 degree. This is way more than the 70 degree used during print. It looks like this has been going on for hours. 
octoprint says target bed temperature is off.
I move the heated bed cable bundle and find the heated bed start losing temperature.
What can possibly be the cause of this?
No clue, sounds strange. Are you using an SSR or anything fancy to power the bed?
I use an atx psu just like printrbot suggests.
Was OctoPrint reporting 70 target and 100 actual? That would be spooky. If it was reporting 100/100 then I’d double check the G-code.
100/100
Then check your G-code. If it doesn’t have 100 degrees bed temperature in it anywhere then maybe you’ve got an OctoPrint or Marlin problem.
Check the end code. You will have a start code, home, set extruder temp, etc. there is end code as well, turn everything off. Perhaps there is an error here?
Which slicer you used - see its setting for bed per type of plastic. It seems that bed temp was switched.
see gcode file for value
http://reprap.org/wiki/G-code#M104:_Set_Extruder_Temperature
I don’t think I will ever know what the cause of the problem. I noticed that my spool fall off its stand so it could have potentially knock loose the heated bed power cable. I do think that the next revision of Printrbot Simple Metal should have the heated bed cable connection to the board much more rigid than the current screw-in terminals.