A piece of frivolous
speculation:
The Linear Pottery people, for whom the
Danube was simply "the Great River" (*daxneu-jo-s) migrated
into the plains north of the Carpathians and soon reached Ukraine, occasionally
giving some of the larger local rivers names based on
*daxn(u)-. Some of their descendants explored the Pontic coast
and discovered a huge river that emptied into the Black Sea. They coined a name
for it, "the Mighty" (*ishros > Thracian
Istros), since of course they couldn't know that their
ancestors had once lived several hundred miles inland up the same river. Thus
the Danube was given its second IE name. ;)
Piotr