Nikon D5200 didn't click 999 photos with inbuilt intervalometer. Why?

OK, I found that there's a built in intervalometer in D5200.
I did the set up a night for star trail photography. Camera's battery was full, memory card was empty (16 GB). Now I set the no. Of shots to 999 with interval of 6 secs each.
My intention was to take as many shots as I can till the battery dies. But to my surprise, the camera stopped taking photos after some 160-170 shots. I dunno what happened. Battery wasn't exhausted, there was space in the memory card. Then I again started taking the photos.
Can anybody help me understand that why the camera didn't click 999 shots or clicked photos till battery died or till memory card got full?

If it's making clicking noises, send it for repairs back to Nikon.

The word is "take" not "click" - CAMERAS TAKE PHOTOS!

Read your camera user manual, I'm sure most of the answers are in there. If not, then call Nikon for support. Also make sure you format the memory card in your camera before you start >>>taking<<< photos. If you suspect the memory card might be the problem, change it for a new one.

This is a common problem. Did you have the camera set to auto or manual focus? If it was set to auto and the contrast in scene decreased too low, then the camera may have stopped taking photos because it couldn't confirm that the lens was in focus. This would have been expected in low-light situations.
The interval set, one shot every six seconds, is plenty long enough for the camera to write data to the card, and for the sensor to cool off after each shot. I'd do a web search for this problem. I'm sure you're not the first to experience this with Nikon cameras.

Random thoughts
overheating shutter- you did give it some hammer
camera went into sleep mode 15 minutes after you last touched it

The camera is designed to shut off automatically if the sensor heats up beyond limits.

You can get decent star trails in about 15 minutes of less of continuous shooting in Bulb mode. Just take note that around 15 minutes of shooting will approximate to around another 15 minutes of saving to the memory card.