I'm assuming you couldn't get around the paywall, so here you go:

In May, Twitter’s landlord filed a complaint against its tenant Twitter, alleging that the company is behind on rent payments. The landlord is Lot 2 SBO LLC, affiliated with The John Buck Company, a Chicago-based real estate firm.

The complaint alleges that Twitter set up a letter of credit for $968,000 that the landlord could draw upon if the company failed to pay its rent, and the lease agreement said Twitter must replenish that letter of credit within 10 days if that were to happen.

The landlord used the letter of credit toward the unpaid rent this March, according to the complaint, but Twitter failed to restore the letter of credit within 10 days, as outlined in its lease.

Basically yes, non-payment of rent.