...What does guard the lines entail?
In game terms playing the computer version, you simply choose this late-inning option. What it reflects is the first and third baseman playing closer to the foul lines, hoping to cut off potential doubles. The disadvantage is it makes singles more likely. I don't know for sure, but I suspect it is much more effective at preventing doubles in the game than in real life. I think it's a horrible rule.
I am also only guessing this is what happened. You could email SOM support. I'm curious what they would have to say.