In a healthy organizational culture, the org grows steadily at a pace that's not too quickly, and more senior members acculturate new recruits into the organization. Acculturation can't be rushed; it's the process of transferring all the unconscious, tacit knowledge about how and why things are done the way they are, and because it's unconscious, it happens only through long exposure.

When you have a hiring freeze, the existing culture ossifies and becomes more insular without any new blood. And then when the hiring freeze lifts, you usually need to "catch up" on hiring, which means that you have a sudden rush of new recruits and relatively few senior people. The new people don't have enough old hands to acculturate them, which means that they end up forming their own culture that may be entirely divorced from the existing org culture. Because the new culture outnumbers the old, eventually the old hands leave (taking their institutional knowledge with them), and the dominant org culture reflects that which the newbies brought with them.

Google's culture changed dramatically from 2011-2013, driven by the combination of rapid hiring, a new CEO (this may've been deliberate: if you want to change the culture, one way to do it is bring on a lot of people who agree with you), and the departure of key early employees like Craig Silverstein, Amit Patel, Marissa Mayer, et al.

Come to think of it, the U.S's culture wars are largely a result of the "birth dearth" of Gen X. Instead of a continuous shift in culture, you have two "big" generations of Boomers and Millenials and the latter developed a distinctive culture amongst themselves without regards to existing American culture.