Here is an example of a deadman reaction in MecaStack that is indicated negative vertical reactions.
The negative vertical loads don't actually mean that the guy wire is pushing "downward" on the deadman. What it means is that the tension in the cable has been reduced due to that load case, and so the vertical reaction is reduced. For example, in the above table the pretension produces a vertical reaction upward of 4.48 Kips. If we then consider "Thermal Cold", which is the case where the stack is cold and contracts, the tension on the guy wire is reduced. This results in a negative vertical load of 2.66 Kips (Ref Load # 6). We can't actually have the cable pushing downward on the stack, because a guy wire is a tension only member; however, we do reduce the tension in the guy wires, and so we now have a total vertical load of -0.05 Kips (Dead Load + 4.48 Kips (Pretension) - 2.66 Kips (Thermal Cold) = 1.87 Kips (Upward). We should not see the combination of multiple "Realistic" primary load cases add up to be negative, which would be an indication that the cable has gone into compression. By realistic I mean that we can't combine "Thermal Hot" and "Thermal Cold" together, because the stack couldn't be hot and cold at the same time. Also, Pretension and dead load are present in every load combination, so those would need to be present when adding multiple primary cases.