It looks as if the DNC is taking steps to make sure that they are putting forth candidates they feel have the best shot at winning a General Election in November. Especially in races where they are looking to unseat a vulnerable Republican. It appears they are looking for more moderate candidates, who would appeal to a wider swath of voters in their district.
For example in this case, the candidate Hoyer asked to drop out, is looking to pursue an agenda of Medicare for All, Free College for All, and Impeachment of Trump. The DNC seems to not want to support candidates of this nature, in their hopes of retaking the House in November.
A lot of posters here accuse the Democrats of seeking to only run on the notion of Impeachment of Trump, but it appears the national party is not interested in this pursuit. At least as a campaign tool. Now, should they reclaim the House, who knows what they will do.