You mention that the tank is full of water but as it is in bypass mode, it would at first seem that there should be no pressure on the tank to push any water out.
The water in the tank was no doubt initially cooler than the surrounding air temperature at the softener's location. Since water will expand as it warms, the contained water will have expanded in volume over the course of hours, thereby requiring additional space as liquid cannot be compressed. Since the unit is bypassed, there is nowhere for the increased volume to expand to.
I don't know for certain that Fleck has designed an over pressure relief into the 7000 valve, but it would seem to be necessary otherwise, the weakest component would be damaged by the force of the expanding water. Since the tank is probably the thinnest component with the largest surface area, it could likely be the tank which would split.
Until you place the unit online, you may wish to advance the softener valve to the 'Backwash' position and then pull the power plug, thereby holding the valve open to allow expanding water to freely drain from the drain fitting while also allowing air to enter the tank as needed if the surrounding air should cool and the contained water contract somewhat.