If you have Google Analytics installed, I'd recommend looking into the medium/source.
If nothing stands out there, then you'll have to dig a little deeper. To check for some bot/spam traffic, take a look into device, OS, geo, browser, and browser version to see if you can find a spike in a certain combination of those. (Apply secondary dimensions to dig deeper here.) Also check out your landing page(s) for this traffic too (applying an advanced segment could be useful here).
After you find out these details but still are left without an answer, you may want to try searching online for others who may have experienced similar traffic (now that you know the finer details) and you may find an answer there.
Ex. AdRoll was associated with a a big jump in traffic last year: https://www.seroundtable.com/adroll-invalid-traffic-18922.ht...