In most mainstream secondaries, I would say the very bottom set is usually students who have SEN or other challenges that prevent them from fully accessing the curriculum, and they need additional support. Often, they're a much smaller group than others, and schools will try to have a TA present, so there is a lot more adult support than in other groups. In the past, although less so now, there may be more than one TA present due to ECHPs.
(obviously not all students with SEN will be in bottom sets, but nearly all students with significant challenges will have SEN, whether diagnosed or not).
Because of the small group size and the extra adults, there is often less "low level disruption" than in other groups. What does/can happen is more high level behaviour incidents (e.g. walking out etc) due to frustration/not being able to cope with school and so on.
The students who don't care/don't work tend to end up in a lower middle set, and yes, that can have significant behaviour issues. Some of them still end up in middle/upper middle sets, because at least in KS3, a naturally able child who doesn't do much work can still perform reasonably well at assessment- and at this age, very few children do no work at all without there being significant issues at play.
In my experience, middle sets tend to be the worst for "low level disruption", although this can be so constant it prevents others from learning.
Some schools will also try the tactic of moving significantly disruptive students into top sets, because they believe the higher level of challenge/being surrounded by more able/motivated students will cause their behaviour to improve. Of course, this only stands a chance of working if the top sets are actually well behaved. Some students can be very able and very disruptive at the same time.