Sometimes it's the course and conditions. Perhaps it's a different course, or it's a different time of day. I know for me, I mostly play in the afternoon or evening, but just about all tournaments start early (by necessity). Thus, I'm up early and playing when I'm normally not. Sometimes I just don't get my act together - then it's a tough day. But I always have fun.
You can also be affected by the people you play with - nervous because you don't want to embarrass yourself, or too chilled because you play with them all the time in casual rounds.
You can be nervous about every stroke counting vs what you think other people will do. Sometimes when you bogey a hole, you get into the mentality that you need to birdie a hole to "get it back". Just accept that you're not getting it back. No matter how many birdies or aces you get after that bogey, you still have the bogey on your card. That should allow you to play smart and take what the course gives you.
Some holes you may seldom birdie, but come tourney time, you expect it and go for it all. Perhaps it's better to just throw a "good" shot and try and par out and go for the lower hanging fruit (better birdie opportunities).
Some people just plain choke in competition.
I know the common advise is to relax and have fun - it's true that if you're relaxed you'll play better and if you're having fun, you'll be more relaxed, but you have to keep that to the between the shot time. You have to play to win and focus on your game while you're throwing. It's easy for me to get too casual and then I'm done.
One thing to remember is that everybody is playing the course when you are, so if the weather is bad, everybody is facing the same conditions. Scores will not be as good as normal, so just play the best you can and accept it.
lastly, remember that some folks play through the winter while others don't. Those that play through the winter keep their form and "groove", so will probably play more consistently in the first tournaments of the year. Those that hibernate for the winter will require some time to get their swing back - and their putting.