How unexpected were the Michelson-Morley experiment results? Did physicists have theoretical reasons to predict that the speed of light would result to be invariant?
It was a completely unexpected result at the time. The principle of the MM experiment hinged on the hypothesis that Maxwell's equations of electromagnetism were valid only in a special frame of reference called the aether frame.The speed of light was equal to its standard value only in this frame and its speed in any other inertial frame had to be given by the Galilean velocity transformation.It was thought that the aether was an all pervasive medium through which the Earth moved. So the claim was that, by the Galilean velocity transformation, the speed of light measured on the Earth should vary with the direction in which light traveled. This was what the Michelson-Morley experiment failed to establish. The negative result led to the postulate of the special theory of relativity that the speed of light is the same in all inertial frames.
As far as I know, the only clue at the time that the speed of light would be invariant were Maxwell's Equations where "something" shows up as a constant. However, speed of light being invariant in all inertial reference frames is very counter-intuitive. One might rather expect physics to be slightly different in different frames, which is what the MM experiment was looking for.