It's likely a combination of several things. I've never spoken to anyone from there, unfortunately, but I do know a few things about them.
I know that the kids down there have many resources in math and science. My friends who did TMSCA back in middle school told me that there was always this one school (or two, I can't remember) named something like "The Colony" that would dominate the State competition. They start their kids in advanced math early, through harder contests than TMSCA, such as MATHCOUNTS. I know this because one day, I was reading articles on AoPS, and came across this article:
http://www.artofproblemsolving.com/Reso ... mathcountsI learned that Bobby Shen was a MATHCOUNTS champion in middle school, and several other kids from Sugar Land (Dulles/Clements) have come from those middle schools and continued being great at math and science competitions over the past few years. I haven't looked very deep into it, but I am 100% sure that many of the really good Dulles/Clements people have gone through the amazing MATHCOUNTS program down in Sugar Land and have emerged ready from competitions of the AMC series and others. Not all of them have been the great competitor that Bobby Shen has been, but many competed on his team and others still are doing well in math competitions today.
I made AIME this year (hooray) after learning about it last year. I saw this year's AMC 12B results and as my coach scrolled through them, I saw many familiar names from Sugar Land through UIL that qualified as well. By preparing them in middle school through MATHCOUNTS, the coaches and parents down there have created a mass of students that are ready for competitions like these, and can easily go in and dominate UIL and TMSCA. UIL is child's play to many of them, and they're going to AIME, USAMO, and even in Bobby's case, IMO, an incredibly impressive accomplishment.
It's not only that they've done UIL since elementary; they've been exposed to so much more rigorous and deep math concepts that UIL is covering just the basics for many of them. Sure, they have to practice, for getting experience in the contests, learning NS tricks, calculator number crunchers, etc. But with the background that they already have, it's much easier than going in cold without that background. These kids can do UIL and TMSCA with ease; they qualify for AIME and USAMO, and looking at the USAPHO semifinalist qualifiers, I saw a few of them there as well (I didn't really know much about the USAPHO, or any associated Olympiads really, until recently, besides USAMO. I wish I could've taken the test to see where I would have been.) I'm sure they do the USA Biology Olympiad (I think Bobby made this too?), Chemistry, Physics, you name it. They're good all over the place. It's pretty amazing the kinds of things they do. These kids also do competitions like Intel and Siemens, and are conducting serious research projects (and advancing high in these competitions.) UIL is not the only thing they do.
If you look at the article I linked to, you'll see that the date at the top says 2005, the year in which the Sugar Land team won State in MATHCOUNTS. I'm assuming that the program did not start too long before that. Now look at the Resulter:
http://www.texasmath.org/Resulter.htmlIf you look at NS/MA/CA at 2005, I don't think you'll see anyone from Dulles/Clements. Around 2007 or so, when that 2005 team began to enter UIL, you start seeing Dulles and Clements showing up, and legends such as Mark Zhang and Kevin Chen emerge. Bobby Shen follows soon after. After they started that MATHCOUNTS program, the Sugar Land dynasty began and became what it is today.
I don't know exactly how I learned these things, but I guess after a while I began to slowly figure things out and connect the dots, and now I know why they have such a great program. It's because they're all Asian (kidding.) They've got a big network of coaches, peers, and parents ready to develop their kids from a young age and produce leaders in math, science, and technology in high school, college, and beyond. They've definitely got funding and much support from all around. Unfortunately, most schools and areas don't have these things (like mine) so we aren't always given these opportunities.
Although I hadn't known much about competitions throughout my life, I'm glad I at least got to do UIL. After learning about the forum I began to do well here, and now I'm starting to get into AIME and such. I think it's probably a bit too late for me in my high school math career to go to USAMO, and I won't be able to participate in Intel or Siemens. However, I'm glad I've been able to do UIL, at least. I think many of the people on here are the same way, and UIL has given them an introduction to some sort of advanced math and science. We might not have the resources that Dulles and Clements do, but we can still produce good students, given they are willing enough to put in the effort.