what causes the sun to make a total eclipse into an annular eclipse?

Answer 1

The distance of the moon at the time.

If the moon is close, it will be total.

If it is further away, the eclipse will be annular.

Answer 2

The apparent size of the moon depends on its distance from the earth, which is not constant. During the course of an eclipse, the moon can move away from the earth, reducing its visual angle to the point that it is not large enough to cover the sun completely. There are many annular eclipses that never get to the total phase. An eclipse map will show positions along the shadow path that are total and annular.