Well, there's the direct parasitic cause, generally spironucleus vortens, and then there's the cause in terms of what's happening in your tank. I kept and bred Malawis for years, raised several thousands of them, and I reduced incidents of bloat to virtually nil after I figured out the following two things:
Number 1-- Exactly. It's a common issue with Malawi cichlids, certain individuals are pigs and will overeat-- even when the tank is not overfed overall. Especially true of peacocks and mbuna ime. Any individuals that I saw that were getting fat I'd separate to another tank where I could put them on a diet until they got slim.
Some took more than one or two such rehabs before they'd get over their greedy feeding, but often they'd eventually eat more normal. If not, I'd just keep them in a tank that didn't get fed enough for them to overeat.
Another thing that helps is using flakes or smaller pellets, so everyone gets their fair share but it's harder for any one fish to overeat. Overall, though, with Malawis, it's better to underfeed somewhat than overfeed.
Number 2-- Aggression. Don't think because you don't see it it's not happening. It often takes place at night or when you're not in the room. Fish gets bullied or squares off with another individual and gets beat, this causes stress, which makes them much more susceptible to bloat. Once I learned to identify not just the more obvious signs of bullying or fighting taking place when I wasn't looking, but also more subtle posturing and body language, and started separating bullies or separating combatants from one another I further eliminated cases of bloat.
Just these two things (assuming an already appropriate quality diet) and I virtually eliminated bloat. In fact, while I no longer keep Malawis, I'd gone something like six years without a single case while I was still keeping them. (no cases of bloat with my kapampa, either, but then fronts are not especially prone to it in the first place)
...Also, a fish that's started to bloat is not always a lost cause with
this treatment.