This isnt an issue with the DM ladder. This is a general issue with all ladders. At your first ladder you will start at 1000 elo. If you then join another ladder, it uses your elo from other ladders as base. This is the case for every ladder. This time someone with high RM TG ladder joins the DM TG ladder. But we have seen the same situation happening for someone with high RM TG rating joining the RM 1v1 rating.
This is a thread about all things that are currently wrong with elo. You might wanna read that thread to know all the ins and outs. I do hope that the devs will fix these issues soon.