You are correct that Einstein's paper, published in 1905, was titled "On the Electrodynamics of Moving Bodies" ("Zur Elektrodynamik bewegter Körper" in German). The term "electrodynamics" refers to the study of how electric and magnetic fields interact and behave.
The reason Einstein used the term "electrodynamics" instead of "relativity" in the title of his paper is rooted in the historical context of the time. In the late 19th and early 20th centuries, physicists were primarily concerned with understanding electromagnetism. At that point, James Clerk Maxwell's equations had successfully unified electric and magnetic phenomena, and scientists were fascinated by the implications of these equations for the behavior of light.
Einstein's paper addressed a problem within electrodynamics known as the "luminiferous ether," which was a hypothetical medium thought to be responsible for the propagation of light waves. By considering the behavior of light and electromagnetic waves in different reference frames, Einstein developed his theory of special relativity, which revolutionized our understanding of space, time, and motion.
The connection between electromagnetism and the speed of causality arises from Einstein's postulate that the laws of physics, including the speed of light in a vacuum, are the same in all inertial reference frames. Electromagnetic waves, such as light, are governed by Maxwell's equations and propagate at a constant speed, which we now know is the speed of light. The constancy of the speed of light and its independence from the motion of the observer is a fundamental principle in special relativity.
In summary, while Einstein's paper focused on the electrodynamics of moving bodies, it laid the foundation for the theory of special relativity, which encompasses a broader understanding of space, time, and motion. The connection between electromagnetism and the speed of causality arises from the fundamental principles of special relativity, which revolutionized our understanding of the universe.