Violence first, questions later?!?

haha I wondered the same thing...... guess it's acceptable behavior these days

Maybe she calculated that if he was there with bad intentions, he looked significantly stronger than her, could be armed, etc, so if she'd just approached and asked what he was doing there, it would give him an opportunity to attack. So it was better to use the element of surprise to incapacitate him and then ask questions. Or maybe it was instinct taking over - she saw someone in her cabin who shouldn't be there and went at him before she had time to think. Speaking of time to think, I'm clearly giving this way too much thought ha ha!

I just assumed that was a romance novel/Hallmark movie trope?

having just watched again for the first time in years, I've got to say I'm with the OP on this one. It is a ridiculous moment, and only makes the female lead come across as unhinged.
She'd literally only been at the cabin for a few minutes and left the door unlocked - like the OP says, this guy could have been an employee. Booting someone in the kneecap could also cause lasting damage, especially a guy that age who could have had a previous injury there. No way would he be walking fine a few minutes later, if they really wanted to play it for laughs they should have had him limping for the rest of the movie as a reminder.
In fact there was no need for the moment at all, it would have been just as 'meet-cute' if they'd gotten into a big argument and then she could have threatened to use martial arts on him - actually this is the only part of their first encounter that gets mentioned again, so perhaps the film's writers realised the assault was too much and figured they'd re-shoot that scene again but then forgot to do so or ran out of time.
TLDR - please disregard my earlier reply - this moment was enough to ruin an otherwise cute movie and I can only imagine the backlash if it had happened the other way around