It's no coincidence at all. The story of the Mexican style of music you're talking about had its origins in central Texas around 1830 when a few immigrants established the first German settlement. The word about Texas spread back home, and within a few years formal efforts were under way to help Germans establish themselves in what would become known as the German Belt.
At the time and even now, to a certain extent the Río Grande marked more of a political and geographical divide than a cultural one, and the musical styles of the immigrants became popular among those of Mexican heritage. One of the most important musical instruments of the Germans' musical style, the accordion, became especially popular and was frequently used in dance music such as waltzes and polkas.
Today, various overlapping styles of music that descended in part from the German music include tejano (from the Spanish name for Texas, Tejas), conjunto (which features the accordion along with the bajo sexto, similar to a 12-string guitar), Tex-Mex, quebradita (heavy on the horns), banda (similar to the polka), ranchera, norteño and various mixtures of the above. The musical style also has influenced music from other parts of Mexico, such as the mariachi music of the Guadalajara area. Such musical styles are especially popular in northern Mexico and in places of the United States where there is a large immigrant population of Mexican heritage. Incidentally, the music is nearly always performed in Spanish, even by Mexican-Americans who speak primarily English. (Native Texan and crossover artist Selena sang in Spanish as a girl long before she could speak Spanish, which she learned to market herself better in Mexico.)
So common is the tejano-style genre that in the United States it is often erroneously viewed as synonymous with Mexican music (or even with Spanish-language music). In fact, though, Mexican music these days is incredibly diverse. Although you'll hear tejano on Mexico City radio stations, you'll also hear Mexican-produced rock o rap en español as well as the same English-language hits you can hear across the border to the north. Spanish-language versions of songs by international performers such as Enrique Iglesias and Shakira are also quite popular.