Lyxen
Great Old One
So just to clarify your position. According to you I can ready an action to move 20 feet if a thunderous boom begins to happen, but before it reaches me.
No, because you need a perceivable circumstance to happen and to finish before your reaction kicks in. So you could not move away from a fireball explosion, for example, because the explosion would be the perceivable event and it would need to finish for you to use your reaction, and when it finishes, you are already burnt.
HOWEVER, if you were watching for the bright streak from a pointing finger as a triggering event, a DM might let you use it as a trigger since you are basically spending your entire action watching for something perceivable that happens before the explosion ("...and then blossoms with a low roar into an explosion of flame"). Since it's sequential, the bright streak is perceivable and it finishes before the explosion start, so you can use as a trigger to avoid the explosion.
And, as put forward in another answer, it's just the resolution that is sequential, the description is that the target is so much watching for the streak to happen that he is almost already moving when the streak appears. A truly heroic dodge, but you must be sure of what the other guy is going to cast, and you just basically wasted your whole turn to do that. But if a caster is repetitive enough, then he will give chances to his opponents, which again encourages creativeness.
Then if the caster uses Thunder Step I can interrupt because the trigger happened and blithely move out of the way, taking no damage?
If you are watching for the guy to disappear, not if you are watching for the boom. And again, that is taking an incredible chance, but it can pay.