Many operations in MATLAB, when it would be beneficial to use multiple threads in their computation, are multithreaded even without Parallel Computing Toolbox. Generally this means you're operating on a large enough set of data.
If you wanted MATLAB to not multithread you could start it with the -singleCompThread startup option or call the maxNumCompThreads function. Note that each of these affect computational threads; MATLAB can use non-computational threads (for things like the Desktop or graphics operations) even if you've limited the number of computational threads.
