Why do crises lead to restrictions of civil liberties and rights?

Crises can lead to a diminution or suspension of civil rights and liberties because there is always a tradeoff between these and safety and security.  A democracy is always trying to strike a good balance between them, and in a crisis, the pendulum almost inevitably tips towards safety and security. 
The events of 9/11 are an excellent example of this.  The Patriot Act, which was passed by Congress in response to 9/11, was meant to make us feel safer and more secure, and in the process, we gave up various civil rights and liberties.  In particular, we gave up our right to privacy, allowing various agencies almost carte blanche to watch all of us in numerous ways, our banking transactions, our phone conversations, and even what books we check out from the library.  The pendulum tilted strongly away from our rights and liberties, and I see little sign that it is tilting back.
The European Union's (EU) response to the immigration crisis in conjunction with ISIS is another example.  The idea that one could travel freely from one country to the next and work in any country in the EU is likely to go in the dustbin of history soon.  Borders are closing, the United Kingdom is in the process of leaving the EU, and it won't be long before passports are needed from one country to the next as well as a bureaucracy meant to make getting working papers difficult or impossible.  The right to travel and the liberty of working are in the process of being sacrificed to safety and security. 
It is understandable that this would happen, given the normal human desire to be safe and secure.  But there are a number of troubling aspects to this.  First, there is no such thing as perfect safety and security, no matter how many rights are lost.  Second, there are administrations that use crises as an excuse to take away people's rights and liberties, which is happening as we speak in Turkey.  Third, once these are taken away, it seems it is very difficult to retrieve them.