When the disciples saw it, that is, when they saw what happened to the tree.

Marveled (Good News Translation “were astonished”) translates a verb made from the same stem as the noun translated “wonderful things” in verse 15. The verb is first used in 8.10 (see comment there).

The disciples ask the question How, but “What caused” or “Why” may be more natural for some translators.

In the question of the disciples, at once (Good News Translation “so quickly”) is in the emphatic position. It is the way the curse was fulfilled immediately which astounded the disciples.

Quoted with permission from Newman, Barclay M. and Stine, Philip C. A Handbook on the Gospel of Matthew. (UBS Handbook Series). New York: UBS, 1988. For this and other handbooks for translators see here .