I can't give you a historical perspective, but I do have a few things for you to consider.

Firstly, tracing the history of any martial art is far more difficult than anyone will tell you. There are usually gaps in time scales, verifiable documents are held in private hands, and there are major incentives for embellishment or forgery. There were episodic periods where many historical documents and monuments were destroyed in china. In other words, the older something is, the more likely it was forgotten, or the surviving information is unreliable. It may be possible that whatever documents you are looking at that tell you there was a surge in Tai Chi practitioners simply don't account for pre-1800s Masters that were forgotten.

Secondly, tactically Tai Chi is designed for advanced Martial artists. Modern day Tai Chi is treated like a kind of socially acceptable calisthenics. It's purpose in defense is to reinforce subtle body movements that only evolve after the practitioner has already been training for years, therefore it's considered to take at least eight years at a minimum to become proficient at Tai Chi.

The 1800's were rough times to be alive in China. There was a level of violence that rivals the middle east today. Guns were also rare items. I would theorize that there was an increase in many different kinds of martial arts just as a matter of survival.