Sometimes I think it has to do with whether there's an organized tour of people who know each other.
I was part of a youth choir that traveled via air. There were about 20 of us plus 6 adults. Each time when we landed back at home -- LAX -- we broke into applause. I think it was due to the fact that we were actually glad to be home after 3 weeks away -- especially on the trip home to LAX from South America. The other was a flight from Colorado back to LAX. (Maybe other pax were applauding because they were glad to be done traveling with a group of kids?! Though we were pretty well-behaved....)
Since then, I've accompanied my sons on school field trips from SAN to SAC. Again about 30 kids and 15 adults in the group, and again the plane broke into applause when we landed. I think the kids were excited to be on the fun field trip, and I think the other pax were glad the flight with the obnoxious kids was over. (That's the way I felt and I was one of the chaperones!)
I've been on other flights that have had school groups or youth sports teams flying (but have not been part of the group/team -- just a private pax) -- usually to HI. Applause happens when landing on either leg of the trip and I think it's the excitement of traveling in a large group of friends.
Also remember one flight with lots of turbulence and a rough landing where the pax clapped. I'm sure it was relief that the ride was over!