I am not sure but here is a guess: Bypass closed and hot water heater electric left ON, pressure builds up and pushes the water back into the supply side, has to find a place to go so it finds the valve with the least resistance to holding it back. fills the bowl up. So look to see if the hot water heater is on electric, the switch is "ON" and look to see if the bypass valves are set to bypass. Otherwise there is no possible connection between the water heater and the toilet.
No man has a good enough memory to be a successful liar. Abraham Lincoln
2016 36FBTS Pinnacle
2016 F350, 6.7, 4x4, DRW, long bed
B & W Companion 5th wheel Hitch