Why did the Spanish feel threatened by the French in East Texas?

I did not know they did.

The French and Spaniards (not Spanish) had a common boundary in North America; but this boundary was never materialised nor patrolled.

The French held theoretical sway on all lands draining into the Mississippi from the Great Lakes to Louisiana. This was the result of Cavelier de La Salle trip down the river in 1682. The Spaniards claimed to their rule all territory north of Mexico as subjugated by Hernan Cortez, De Soto and other Conquistadores. The two vast domains obviously had to butt somewhere in Texas or New Mexico; yet no one was there to stake a land that belonged to Sioux and Cheyenne tribes or, to less quarrelsome Indios Pueblos.

The English colonists had more problems to the South of Georgia and the Carolinas since Spain occupied Florida. The hamlet of San Augustìn was highly disputed and changed hands several times.

Louisiana too changed hands (before being purchased by Jefferson) between Spaniards and French. But that is not what I would call East Texas.

I do not think Texas ever was much of an issue (before 1848 between Mexico and the US). But I might be wrong.