Originally Posted by C. W. Fields
I think Caro has a certain mental ascendancy over Mona. Mona had beaten Kerber previously so she really believed in herself in their 2R match while she has yet to take a set off Caro. Believing in yourself is key - especially when your psyche is somewhat wobbly - and while Caro always expects to win before a match (at least that's what she says), I don't think Mona fully believed she was gonna win today.
I think that Mona was probably expecting no pressure from Caro and thought she would prevail easily with her attacking game. I think that there might be this belief in the tennis community that the defensive style is less effective by default. But the trend when you look at the top 10 shows something different. The most successful players nowadays are those who can play excellent defense (although they are not completely reliant on that)