Yeah, I must be missing something here. Are those who do not get vaccinated putting those of us who are at serious risk?
Yes. The measles herd immunity threshold for the MMR vaccine is 92-94%. If more than 6% of the idiots around you go unvaccinated, measles becomes likely to spread among people who have already taken the vaccine or otherwise acquired immunity.
The reason is simple: the immune system is random. The B cells in each vaccinated individual produce different antibodies in response to the same antigen. Since an antibody's response to antigen X1 doesn't correlate much with its response to antigen X2, and different lines of a disease have different antigens, no vaccine can be 100% effective. Any one person might have total immunity to some given line of the disease (called a "quasispecies"), yet be totally vulnerable to some other quasispecies whose antigens are invisible to the existing antibodies. Different people are vulnerable to different quasispecies, and there are thousands of quasispecies (grouped into 21 strains in the case of measles), so we usually just throw our hands up in the air and pretend that infection vulnerability is a wholly non-deterministic thing.
Herd immunity is the threshold where each infection produces, on average, one new infection. If the vaccination rate is above herd immunity, each infection produces less than one new infection (exponential decay). The outbreak reaches its peak quickly, then vanishes as the existing victims fight off the disease (or die). If the vaccination rate is below herd immunity, then each infection leads to more than one new infection (exponential growth). The outbreak then grows rapidly until so many people are already carrying the disease that the disease runs out of new hosts, reaching a new steady-state of one new infection per infection... at which point we say it has transformed from epidemic (an outbreak) to endemic (never going away on its own).
If vaccines were 100% effective, falling below the herd immunity threshold wouldn't be so worrisome for people who are vaccinated. True, among vaccine-refusing populations (and those who can't benefit from vaccines, e.g. babies, the very elderly, AIDS patients, and organ transplant recipients) the disease would perpetually rage, as there would be enough contact between vulnerable islands that the disease never quite burns out. But in reality (a) each person who is immunized has a small-but-nonzero chance of catching the infection (and passing it on), so everyone is potential virus-habitat regardless of vaccination status, and (b) more victims means larger viral population means more viral reproduction means creation of more quasispecies. More quasispecies means that, if there is some way that the antigens can change that will give the disease access to new victims without compromising the disease's ability to spread, evolution will find and exploit it sooner rather than later, so the virus can get its grubby little capsid proteins on fresh meat that other strains can't touch (i.e. you).
What we're seeing in Texas is an outbreak in an overall US population where vaccination rates are falling, but still above the herd immunity threshold... for now. If rates continue to fall, we can expect these outbreaks to become larger and more frequent, until they eventually reach criticality and the end of one outbreak always overlaps the beginning of the next, i.e. the disease becomes endemic again.
(Pertussis is also stupid contagious and thus has a high threshold for herd immunity, but pertussis is about 10 times more likely to kill a baby than measles is. Like measles, pertussis is also seeing big ugly outbreaks these days: the Denver metro area, Northern California around Marin, Washington state, i.e. basically the places where the cultish and vaccine-refusing Waldorf School has a notable presence. Annoyingly enough, the DPT and TDaP vaccines was never even implicated in the original Wakefield autism-vaccine nonsense, yet the vaccination rates have been falling about as dramatically as those of MMR, probably because Wakefield's "MMR is bad (and here's a patented replacement vaccine, no payola I promise!)" got simplified into "vaccines are bad" in the US's celebrity-worshipping mass media echo chamber.)