Watered down DEF can cause a DEF derate. A bad NOX sensor can cause it. Also, a cracked DPF filter can do it. There are no doubt other possible causes as well.
This is one reason I recommend buying the DDDL software and the NextIQ interface. A forced regen will sometimes clear the bad quality DEF code so you aren't derated, even if you haven't fixed anything yet. It might derate again during the next driving regen, but you'll get down the road until then. With DDDL you can also set the parameters that determine what happens during a derate. You can't stop a derate from happening altogether, but you can change the derate speeds from 55/45/5 to something like 95/94/93. That way you just get the blinking light until you fix the problem. There is also a derate acceleration limit that defaults to 0.5 kph per second (2016 Cascadia DD15). You can set that to 100 or something so you don't feel it.
I have a 17' Cascadia with DD15 and once had the exact same thing happen after a dpf cleaning. The truck went maybe 10 miles at 55 then derated to 5 mph. Had to get towed to Amarillo Freightliner shop . Shop informed me dpf sensor had failed probably due to dpf cleaning.
Apparently Detroit doesn't recommend dpf cleaning only replacing filter.
It's safer to swap your filters for a pair of reman filters because it avoids the risk of having your filters crack during cleaning. Detroit takes the risk instead of you. It costs of course. Cleaning is only about $400. Remans cost about $1100 plus another grand for a core charge. On the other hand, if they break during cleaning you're out the $400, plus over two grand for a set of remans that you have no cores to exchange for.