teaching - 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.