Why is it not possible to prevent sudden unemployment of thousands of people? Why is it not possible to prevent sudden unemployment of thousands of people? 
Often one can observe, for example in news, that suddenly from one day to the other thousands of people of a company get unemployed. Why is it not possible to prevent this or why do companies not downsize slowly instead of above stated? Of course those companies should know this earlier and they could downsize slowly, but they don't. Why?
I would really appreciate any solutions and ideas
This question boils down to asking why business cycles exist and change employment in shocks.

One possibility is that employment is sticky or not worth changing constantly. A firm cannot constantly calculate and evaluate worker productivity and general profitability without a, perhaps rather expensive, cost, especially if the firm is very large. Constantly hiring and firing to try and have "optimal" employment for a firm is also bad for morale.

And perhaps more to the point, the alternative to firing people in large swathes is lowering wages, which is even worse for worker morale and sometimes not really legal (e.g. salaries are fixed for a year). So if there is a recession for example, a firm will generally choose to keep wages up, and then when it can't afford such a scenario anymore, it will create large layoffs.

Further Reading: The cost of firing changes the dynamics of how people are fired as well. See this article about France and Spain's different changes in employment with respect to the Great Recession. It is possible to prevent large drops in unemployment. Whether it is efficient is another question. My answer only briefly touches on the theory, and there is plenty of literature related to the points I've made that you can look for.