Why does China has to rack up so much debt now?

I am amazed by China's economic history since Deng Xiaoping, and I still have a hard time wrapping my mind about the fact that in roughly 50 years China went from "nothing" to being the leader in many economic segments/markets.

I've also seen how entering the WTO helped them to increase their exports and money flowing in, and they managed to build a roughly $4trillion foreign reserve currency back in 2013, the highest so far.

But despite all this economic success, scale, improvements in technology, reliability, efficiency, productivity, education, patents and all, it seems China hit an invisible barrier in 2014 and never really got over it. Also, their total debt is going up much faster in the 10s and 20s, and to me that screams that something is going wrong for them to end in this situation where growth is slowing, but debt is acellerating.

So my question is: 1-Why does this shift from high growth, small debt and high reserves to low growth high debt and small reserves ocurred?

2-Can they reverse course and stop increasing their debt without deflating their economy even more?