I caught several of my students plagiarizing. Could it be my fault as a teacher? I have been teaching for three years now. This year, for the first time, I caught multiple students in a class (out of 50) either handing in the task of a colleague or copying the majority of someones work.

Now I've started to question myself whether this could be on me in any way. Could I have caused this behaviour by something I said? Am I getting paranoid, or was this just a coincidence or the usual abnormal behaviour of some students?

For more context: All students in this class are going to be teachers. In the first lecture I mention some e-learning resources including the online library of all master's thesis of the faculty.

Additional information to answer some comments (some comments are now in chat): This is a course at university level. In this University cheating is -- on paper -- taken as a serious offense. Meaning, if you get caught cheating it will be noted in your examination record and you have to repeat the whole class. Apparently, not all students know this.

I mention the online library in this course because we are discussing electronic materials in this course. One topic of the course is using "digital tools" for teaching. For me this includes using existing resources for preparing classes.

I checked my slides from the introduction. I did not have a "do not plagiarise note in them". Although, I believe that this should be not necessary.

I have to admit that I might invite them since I do not change the tasks between semesters. The reason for this is that I quite like them and that I have the feeling that the output of the tasks will be useful for them in the future.
I suspect that in a group of 50 students there will be a few who want to cut corners. It isn't your fault, exactly, but there are some things you can do to make it less likely. If the  number is small, you can deal with it individually in your office, of course. 

But you should consider why people feel that cheating of any kind is a viable option for them. 

If the tasks you set are all very high risk then people will sometimes act badly out of fear. This goes for both assignments and for exams. If you permit re-work to improve grading on assignments you lower the risk and improve the chances of proper behavior. 

If the risk of cheating is extremely low, some will do it out of laziness. I once had a group whose experience previously was that no one actually looked at their work, so it didn't matter much what was turned in. I had to convince them (and the Dean) that I was willing to fail everyone if they kept up that behavior, and also convince them that I would look at and comment on their work. But if they don't get individual feedback, their work actually has little value to them for learning. With 50 it may be difficult to give this feedback, of course, though it is (IMO) essential. 

Some students have gotten the idea that the reason that you set a task is to get the "proper" answer, rather than to help them learn. So their focus becomes getting that answer, even if no learning occurs. You need to find ways to educate them about the nature of education - especially as they will be teachers. It sometimes surprised a few of my students that I didn't ask them to do things because I needed the answers. I could provide my own answers. It was the production of the answer that was important, not the answer, and the answer could actually be wrong if I could use it to educate (re-work, feedback, ...) 

There are other tricks that can be used to lessen the likelihood of plagiarism. Don't used old exercises if you find students turning in old solutions, for example. But permitting, even requiring, teamwork can, in many cases, lower the risk of plagiarism, as well as make education more of a social process. It also helps solve the 50 student problem if students work in groups, or even pairs. 

In case of paired or group work you need a way to do peer evaluation, of course, but it need not be meaningless or threatening. 

There are also strictly punitive measures, though I try to avoid them. Giving zero credit for all parties when plagiarism is encountered can be effective. So can expulsion for repeat offenders.