The thunderclap was caused by Tommy, who brought it on unintentionally and was almost as much scared by his handiwork as Grizel herself.

She and he had been very friendly of late, partly because they shared with McQueen the secret of the frustrated elopement, partly because they both thought that in that curious incident Tommy had behaved in a most disinterested and splendid way.

Grizel had not been sure of it at first, but it had grown on Tommy, he had so thoroughly convinced himself of his intention to get into the train with her at Tilliedrum that her doubts were dispelled--easily dispelled, you say, but the truth must be told, Grizel was very anxious to be rid of them.
And Tommy's were honest convictions, born full grown of a desire for happiness to all.

Had Elspeth discovered how nearly he had deserted her, the same sentiment would have made him swear to her with tears that never should he have gone farther than Tilliedrum, and while he was persuading her he would have persuaded himself.

Then again, when he met Grizel--well, to get him in doubt it would have been necessary to catch him on the way between these two girls.
So Tommy and Grizel were friends, and finding that it hurt the doctor to speak on a certain subject to him, Grizel gave her confidences to Tommy.
She had a fear, which he shared on its being explained to him, that she might meet a man of the stamp of her father, and grow fond of him before she knew the kind he was, and as even Tommy could not suggest an infallible test which would lay them bare at the first glance, he consented to consult Blinder once more.