Most writers ascribe Apollo's trickery to jealousy.
This particular version of the Orion myth comes from the poet Istros, as preserved by Hygnius in his De Astronomicon. The original Latin text is as follows:
Istrus autem dicit Oriona a Diana esse dilectum et paene factum ut ei nupsisse existimaretur; quod cum Apollo aegre ferret et saepe eam obiurgans nihil egisset, natantis Orionis longe caput solum videri conspicatus, contendit cum Diana eam non posse sagittam mittere ad id quod nigrum in mari videretur. Quae se cum vellet in eo studio maxime artificcm dici, sagitta missa, caput Orionis traiecit.
Here's a slightly paraphrased translation by Joseph Fontenrose, who was a classics professor at UC Berkley:
In Istros' version, Artemis killed Orion with her arrows, though she had no intention of doing so - she loved him and intended to marry him. The jealous Apollo often chided her to no avail about her love for Orion. One day when she stood with her beside the sea he noticed Orion swimming far out; only his head was visible, a mere speck on the distant sea. Knowing well his sister's skill with the bow, he cunningly expressed a doubt that she could hit the distant black spot with an arrow. Artemis accepted the challenge and thus killed Orion.
He then summaries:
Apollo's jealousy of Orion seems motivated by his own love for Artemis.
Fontenrose, Joseph Eddy. Orion: the Myth of the Hunter and the Huntress. University of California Press, 1981.
Make of this how you will, but one interpretation is that:
Her brother, Apollo, was jealous of Artemis' love for Orion, a great hunter . . . This pictures how relationships can be destroyed in the Artemis woman through the jealousy of the spiritual animus, here signified by Apollo. It is as if the woman already has a partner within her own psyche which wants no competitors from the human realm.
Edinger, Edward F. The Eternal Drama: The Inner Meaning of Greek Mythology. Shambhala Publications, 2001.