I'll attempt to answer the question more from an engineering point of view than an historical one, and see what conclusion we reach.

First thing first: As with a lot of other info in Wikipedia the 171k number seems vastly over-inflated and comes from a new yorker article, every academic paper i have found on the subject moves in the 25-85k range. The event consisted not only on the failure of the Banqiao dam (that being the largest) but on the failure of other 61 dams, from large (like Banqiao and Shimandan) to 58 small dam failures.

The average annual rainfall of the region is about 1000mm per year, but during the storm there was a single day rainfall of 1060mm, and during the next 5 days the accumulated rainfall was 1600mm. This amount of rain is consistent with a 1 in 10,000 year flood (statistically we expect it to happen once every 10,000 years) and the Banqiao dam was designed with a 1 in 1,000 year flood in mind, the rainfall greatly exceeding this amount. Other dams were designed with even smaller floods in mind, the Shimadan dam had a 1 in 500 year flood design and most of the small dams were designed for a 1 in 50 or 1 in 100 flood. From the engineering point of view, so far, we can say that mother nature was the cause since the design periods seem acceptable for the magnitude of the dam.

However, negligence took a part on the failure. After the fact it was found that the Banqiao dam had an overstorage of 32 million m^3 before flood season, in a region where most rains come during this period this is reckless and limited the capacity of the dam for flood control. It was also found that poor maintenance of the sluice gate greatly diminished their capacity (since they were filled with slit), so soon it reached a point that more water came into the reserve that could get out.

In conclusion, even in the best case scenario with no human negligence the chances of the dam failing were quite high, since the rain greatly exceeded what anyone expected, however the fact that negligence DID happen only increased the chances of this failure.

Lack of contingency plans made the number of lives lost much higher than it should be, the authorities had several days to issue evacuation plans to all the zones that were at risk, but there was no plan in place for this warnings. A lack of understanding of the authorities on the chances of the Banqiao dam failing ( it was tough as an indestructible dam) made them have a false sense of security and concentrate on other troubled areas while ignoring the most critical one, no one expected the Banqiao dam to fail.

Sources: "The World’s most catastrophic dam failures." Si Yi.

"Lessons from Catastrophic Dam Failures in August 1975 in Zhumadian, China " Yao Xu et al.