Just trying to work with this off the top of my head...
It seems to me that first and primary cause to the desire to change things is to meet new distance and B&I performance enhancements by changing the field of play from what was previously designed to the highest and most widespread B&I of that original era when the particular course was first built. Whether that original course was built in 1890 to the best B&I of that time, or in 1925 and built new at that time and correlating to the best B&I of that day. When a new equipment advantage came out, the previous era designs obsoleted to some degree that caused those members of the older clubs to look at the most recent top designs that were responding to the changes, and pine for course characteristics like the newest designs. So, that could have meant an original architect's work from 1910 being obsoleted and thought to be behind the curve to the very same architects newest designs some 20 years later. So, what the archie learned in the intervening 20 years of keeping up with the latest B&I and competitive and craftsmanship improvements were desired by the older club to integrate and modernize to what the new perception of 'the best' had become. That process can be placed in any 20-30 year era since B&I improvements and also construction equipment improvements always happended in any segment of 20-30 year passing eras.
The next thing that contributed to the rationale to tamper with original designs was aesthetic fad of what is cool looking besides golf functional. As parkland courses proliferated, trees and inland terraine features were more artfully or aesthestically integrated into the most modern designs, and the herd mentality of the older clubs felt like they had to have some of that as well. The same with water features. The more water features were integrated into more modern designs, the more the older clubs without the water features felt like they had to have some of that too. They all began to crave the 'signature hole' look, as a "keeping up with the Jone's club" mentality. So, whether the new aesthetics fit good golf design playability sense or not, they had to get some of that.
Then, as noted above, the actual need to redesign for lack of good understanding of engineering aspects mostly of drainage caused many a remodel that just evolved contrary to original design concepts.
It seems to me that there is no one or even tidy set of rationale or factors as to why GCA is subject to fads. Some fad is response to functionality and well conceived, and some is superficial and owing more to the herd mentality of a club organization that is inherently responsive to peer group pressure to keep up with what they percieve as 'the best' despite if it makes good golf sense or not. Each club or each golf course management/ownership entity has its own culture and value set that tend to respond to changing circumstances in their own unique way. The only thing constant it seems is that issues extraneous to the golf course as a physical thing as originally designed, changes.