The gerrymandering has to include just enough voting Republicans in a district to make it safe for the Republican incumbent. They try to crowd all the Democrats into a smaller number of districts.
But people move. People change parties. People have kids. In the event of poor Republican turnout, many of those "safe" districts are not safe at all. Because they tried to maximize their number of districts, just 5% missing voters could turn them Democratic.
I think the Republican civil war could make for this kind of surprise next month.
