Why did the Mexican troops feel justified in attacking Zachary Taylors troops?

There is no evidence to suggest that Mexican troops felt justified in attacking Zachary Taylor's troops. While border disputes in the Rio Grande Valley region, including the town of Brownsville, Texas, can be attributed to the Texas Annexation and its aftermath, the main events leading to the outbreak of the war were the annexation of Texas and disputes over the location of the US-Mexico border following the annexation of Texas.