Heart ailments are not that rare in young adults; I mean, most of us will never experience them, but they aren't exactly rare, either. I'm not surprised that some of the riders discover their arrhythmias and other heart diseases in their 20s.
Are some doping related? Possibly, I don't know. I would be surprised if they were all doping-related, because one would expect to find a number of cases in a group of this sizes given the overall incidence rate. Moreover, some that might never have been diagnosed in the general population, might be diagnosed in the population of pro cycling precisely because they are under regular medical scrutiny.
Is the observation of a cluster enough to detect a systematic increase? Not really, in a random sequence, clusters happen. (In fact, a very regular rhythm or pattern is probably not random.) Besides, without a careful analysis, we don't even know if the current occurring numbers are unusual.
So, to make a true inference, one would need to carefully analyse the data of the last decade or so to see whether or not a new trend seems to be happening, while controlling for trends in the general population. I would expect the incidence rate to be slightly above the general population incidence rate, as I expect the probability of detection/diagnosis to be higher in the pro cycling population, due to the higher incidence of medical checks in that population compared to the general population.