Note the wording very carefully. It should never say the Pokemon "used Struggle". Struggle doesn't exist as a move in contests, and it cannot unless you hack. Since you haven't hacked, it's not the case that Pokemon are using Struggle, because there are instances where the word "struggle" is still used.
The one example that I know of should say something along the lines of "Mienshao struggled to show off much of its appeal because it wasn't excited enough". This happens when you use a move that works well only when the Pokemon is pumped, e.g. Acrobatics, but you didn't use a move before it to get the Pokemon pumped, e.g. Bulk Up.
I think there's also other instances that the contest text would say "the Pokemon struggled to do [whatever]", but I can't remember them specifically.
You probably clicked too fast and only saw the word "struggle" and misinterpreted.
Source: experience.
