Instead of its orbit parameters showing the normal smooth changes, the orbit started to to show signs of sudden, intermittent, changes that are not characteristic of air drag. There are two possible explanations, one good and one potentially ominous.

The effects could be caused by periodic thruster firings.

Alternatively, the thrusts are not 'voluntary' and the effects are due to something else, such as a permanently-firing attitude control thruster, or venting of gas from inside Tiangong. One potential source is a propellant leak and another is leakage of air from within the cabin. If it were either of these then Tiangong would also likely be tumbling.

Nothing appeared through Chinese news media or the 'normal' sources of rumour about the Chinese programme, and visual observation soon after the behaviour started showed no unusual variation in brightness. Currently, there is nothing to indicate tumbling or rotation so all is probably well and we are looking at a thruster experiment.

Most of Tiangong 1's orbital parameters show unusual behaviour to a greater or lesser degree but the one where it shows best is a parameter called 'ndot2'. It is a measure of the rate at which an orbit is shrinking. For a low orbit, it is almost exclusively controlled by air drag. Effects on the other orbital parameters can be seen here.