Why does Mark (like the other synoptic writers) show Jesus quoting this line at the Temple?
As I said, it’s from Psalm 118. If you read this, you will find that the beginning seems to be stage directions for various groups to speak in the liturgy of a royal entrance to the Temple. Toward the end, “Bind the festal procession with branches up to the horns of the altar,” is probably a reference to the Feast of Tabernacles where the king is supposed to come to the Temple to read what Deuteronomy says about his place in the scheme of things.
If Jesus is referring to a portion of the psalm, his hearers are familiar with the rest. “Blessed be he who enters in the name of the Lord!”–the anointed king, that is–is also from this psalm.
He is implying a claim to royal honors while the priestly hierarchy, their leader appointed by the Romans (and his ceremonial robes kept in Roman custody) feel put on the spot by this demand. Whatever they do is likely to bring trouble, from either the Romans or their own countrymen, whom they are supposed to be keeping in (Roman) order.