When you find an area which has a lot of species belonging to the same genus, there are two simple explanations - either this is the place where the genus diversified, and only a few species have expanded beyond this "centre of origin" or this is a place where there was secondary diversification. So how do you tell the difference? On one level, it's simple - where are the oldest species located? If the species located in the species-rich local area are older than the more widely-ranging species, then chances are this is a centre of origin. On the other hand, if the more widely-ranging species are older, then this is a secondary centre of diversification. In reality, it may be more challenging due to methodological issues, like figuring out what portion of the genome to focus on. In the case of Bursera in Mexico, this area represented a secondary diversification over the last 10-30 million years in a much older genus.
While you think this through, a second question arises - what is the normal pattern of the distribution of species within a genus? Is there a "normal" pattern? I could draw examples from a couple dozen genera, but that wouldn't create a pattern: after all, as they say, the plural of anecdote is not data. Is the distribution of species in genera something that you can generalise to any real extent, or is it too dependent on local factors like competition or the uplift of mountain ranges?