it is indeed due to the wind, so here's stivi's manual for this
- you wanna be constantly checking the wind direction (on any weather app / website), best already a few days before you arrive
- generally, head to the leeward side of the island for quiet conditions. this means, if the wind is coming from the north, you will generally find nicer conditions in the south, whereas the beaches in the north will have choppy / agitated waters.
- it is not just the area of the beach though, but also its orientation that plays an important role. an example: let's say we have wind from the east. logically you wanna head to the westcoast for the calmest water conditions. now open up google maps and check out the area around cala bassa and cala conta, which indeed is in the west. the thing is though, cala bassa actually has a northeast orientation (as in, if you sit on the beach and you look out to the water you are looking towards northeast) and so on a day with wind coming from the east, cala bassa won't be the best. if you drive a little further to cala conta, or even better to raco d'en xic (nowadays falsely called cala escondida), the conditions will be much better as the beach has western orientation.
- on days with a lot of wind, look for beaches on the leeward side of the island, with the correct orientation as explained above and with natural shelter in the form of mountains/hills. another example: we have strong winds from the west, so we wanna go east. if you go to es cavallet which has a very flat land behind the beach, it won't be as nice as if you drive up to aguas blancas which is wonderfully sheltered by cliffs. I appreciate this takes a little more knowledge or previous investigation, but it makes a massive difference.
- lastly, if you've had a few days of strong winds from the same direction and the wind finally stops (or changes direction), don't go seeing those beaches in the previously windward direction on day one. you can be 100% sure the sea will still be choppy, it usually takes a day for the sea to get flat/calm if the conditions really had been rough previously.
hope this helps!