Jonah 4:5

‘Then Jonah went out of the city, and sat on the east side of the city, and there made for himself a shelter, and sat under it in the shade, till he might see what would become of the city.'

Perhaps Jonah had taken YHWH's words as signifying that maybe he was being too impatient when in fact YHWH had plans to deal with Nineveh after all. This is really the only thing that can explain why Jonah went out to a mountain on the east of Nineveh in order to ‘see what would become of the city'. And because it was very hot he made himself a shelter of boughs and leaves, and sat under its shade awaiting events.