No ... but this might be unintended

Channel Divinity to turn undead is not casting a spell or making an attack:

At 2nd level, you gain the ability to channel divine energy directly from your deity, using that energy to fuel magical effects.

As an action, you present your holy symbol and speak a prayer censuring the undead. Each undead that can see or hear you within 30 feet of you must make a Wisdom saving throw...

Channel Divinity is not a spell

If it were casting a spell, the rules would say something like "Using this feature allows you to cast blah blah blah".

From the Player's Handbook, p. 202:

When a character casts any spell, the same basic rules are followed, regardless of the character’s class or the spell’s effects.

Each spell description begins with a block of information, including the spell’s name, level, school of magic, casting time, range, components, and duration. The rest of a spell entry describes the spell’s effect.

The class feature description makes no mention of casting a spell. Further, none of this information is present in the description of Channel Divinity.

And while it isn't an official ruling, Jeremy Crawford, the lead rules designer for D&D, was asked:

Does Dispel Magic auto work vs Channel Divinity powers?

And he opined on twitter that:

A Channel Divinity option like the paladin's Sacred Weapon isn't a spell.

Channel Divinity is not an attack

If it were an attack, the description would say "make an attack roll" or "make a spell attack roll".

From the Player's Handbook, p. 194:

If there’s ever any question whether something you’re doing counts as an attack, the rule is simple: if you’re making an attack roll, you’re making an attack.

How can this make sense?

That's up to your DM. But maybe this part of the Turn Undead feature's description can help:

As an action, you present your holy symbol and speak a prayer censuring the undead.

Your DM might view this as an action that is primarily defensive in nature. In that light it might make sense that it doesn't break the sanctuary.

Or, perhaps it is simply a plea for help from your deity. Maybe it is not you, but your deity that acts. You are merely a vessel to channel that power. Maybe, because you took no direct offensive action, the sanctuary holds.

Or, your group might reasonably agree that this doesn't make sense, and house rule that it breaks sanctuary.

But, ultimately, it's up to your DM to narrate the action.

Is there evidence of the designer's intent?

As Doval pointed out in the comments, Jeremy Crawford has also expressed on Twitter that despite the way the Sanctuary spell was worded, the intent was that other actions should also break the sanctuary. Subsequently, the publisher changed the wording for the Sanctuary spell in the November 2018 Errata.

Before the errata was publish, Jeremy Crawford gave the following responses to questions about the design of Sanctuary:

Example #1:

Q: Cast witch bolt and in the next turn cast sanctuary. The automatic damage from witch bolt cancel sanctuary?

A: RAW: witch bolt exploits a loophole in sanctuary. RAI: witch bolt's damage ends sanctuary.

Example #2:

Q: You first cast spirit guardians and then sanctuary. Does sanctuary end when spirit guardians deals damage to enemies?

A: RAW: Sanctuary isn't stopped if an already-cast spell deals damage. RAI: A damaging activity ends the spell.

November 2018 Errata

As pointed out by BlueMoon93, the Sanctuary spell was changed in the November 2018 Errata:

If the warded creature makes an attack, casts a spell that affects an enemy, or deals damage to another creature, this spell ends.

While this would not change the interaction between Sanctuary and Turn Dead, it would mean that Witch Bolt and Spirit Guardians can now break Sanctuary.