My strider started flying

I was playing a starting in the nether world and I got a strider. I made a little pool of lava for it and I went to the overworld. I came back from the overworld to see my strider briskly start floating upwards, it stopped near the ceiling. I stood there in awe and built a tower to get up to em. (I named him Leo) so I saw leo just standing there in mid air shivering. I got on him with my saddle and I could just fly with him around the nether. I eventually returned to make a floating lava pool for him, and when I put the lava down, Leo flew up another 5 blocks!