( <x-axis rotation>, <y-axis rotation>, <z-axis rotation> )
The first frame is defined by (0,90,0).
The last frame is defined by (90,45,90) which is a rotation about the x-axis from the first frame.
The problem is that from the first frame there is no direct way to produce an x-axis rotation, i.e., a gimball lock position has been encountered.
Interpolating the fixed angle representation produced the animation below.
The viewpoint is from out on the negative z-axis looking towards the origin.
Interpolating fixed angle (or Euler angle) representations can produce unexpected and undesirable effects