These things happen quite often, but T-Mobile has paused an update to Android 6.0 Marshmallow on the Nexus 7 2013 LTE. Now this should only affect those Nexus 7 2013's bought from T-Mobile. The LTE version from Google, that has a T-Mobile SIM card should still be fine. And the images are available as well for your flashing needs. T-Mobile hasn't stated what the issue is, and why they have decided to halt the OTA, but it's likely a small bug that might tamper with their network, and thus causing the roll out to be paused.
On the support page for the Nexus 7 2013 LTE on T-Mobile's website, they do note that they expect the update to begin rolling out again in "early December" which is only a few weeks away. Rest assured that Google, ASUS and T-Mobile are all working on the issue and should have Marshmallow rolling out pretty soon. Reaching out to T-Mobile tells us that their customer service isn't all that sure what's going on either (that's not really a huge surprise though). So we'll just have to sit back and wait on Google and T-Mobile to start the rollout again.
Updates do sometimes get halted. Normally it's due to a bug that is fixed and then the rollout is started once again. And if you have already received Marshmallow on your tablet, from the OTA, don't worry. You will still get the new OTA once it's available. Like we stated before, the factory image is available on Google's site already, so if you really want Marshmallow right away, you can simply download it and flash it to your Nexus 7 2013 LTE. This is the MRA85K build (although the others are just the new security updates, so those should be just fine as well).
Of course, as we always do, we'll be sure to keep you up to date on getting Android 6.0 Marshmallow onto the T-Mobile Nexus 7 2013 LTE. It shouldn't be much longer. But as soon as we have some additional information, we'll be sure to share it with everyone. In the meantime, you can follow T-Mobile's support page, which is in the source links below.