It might help you with your question,
Select Site/Web where your document library is located using these two drop down menu lists which are auto populated when you open speventreceivermanager's exe file as shown in picture below,
Now select your document library from list of libraries on left side,
When you will click on your doc lib, you will see all of the available event receivers for that particular library and if any of event receiver has child nodes then it means library has a event receiver attached to it and then you can either remove that event receiver or look into it (you can note down name of it and then you know which receiver it is) to find out whats happening in the code (you will have to check event receiver's solution which will be somewhere saved by developer of ER)
You might be looking at item added or item adding event receivers specially.