Turkey invaded Afrin over the weekend. The district is the westernmost territory held by the Kurdish YPG, and right on the Turkish border, which has made it a target for some time. Manbij, however, is deeper into Syria, and closer to the rest of YPG territory.
More importantly, Manbij is a city with a US military presence already on the ground. When the Kurds expelled ISIS form Manbij last year, the US put troops there specifically to dissuade Turkey from contesting it, and that’s been a sore spot for Turkey ever since.
Turkish officials are said to believe there is little chance of direct fighting with the US over Manbij, but the fact that they’re willing to take the risk at all is an eye-opener, and suggests Erdogan is more willing to confront the US over their long-standing alliance with the Kurds.
The Turkey-YPG fight leaves the US in a tough spot, with both factions, and the Turkish-backed FSA all US allies. Calls for restraint haven’t done anything, however, and US calls to keep the Turkish invasion “limited” also appear not to be taking effect with the Manbij announcement.