Here are my guesses:
1. Level is relative. Level on the frame may not be level in the tank, and vice-versa. So while you thought you were level, you weren't.
2. Even though you flushed, you still could have had a small "dam" of poo, etc. holding back some water. After sloshing around on your way to the next CG, it entered the drain line, and came out when you removed the cap.
3. All of the above.
1988 Coleman Sequoia - popup (1987-2009) - outlasted 3 Dodge Grand Caravans!
2012 Roo19 - hybrid (2012-2015)
2016 Mini Lite 2503S - tt (2015 - ???)
2011 Traverse LT, 3.6L, FWD
2009 Silverado 1500 Ext Cab, 5.3L, 4x4, 3.73
2016 Silverado 2500HD Dbl Cab, 6.0L 4x4, 4.10