Total sales were up 5.2% for 2015, the lowest increase since 2009, when there was a decrease due to the economic downturn. I would say the reasons are no new niches created for the year (amazing but true) and intense competition in the premium car arena.
I notice that BMW took to SUVs with the X5 in 1999. Now there are five X models, totaling just under 28% of all BMW marque sales. That shows that despite the shift to SUVs, BMW still relies heavily on cars. The X1 and X3 are outsold by the bigger X5, which is the opposite of Audi for example. Perhaps that reflects Audi's strength in Europe and BMW's in North America.
Data source: BMW Group.
Summary: BMW is doing well but Jaguar/Land Rover, Volvo, Lexus and maybe Infiniti are now pushing hard. Add to that the three German brands and the kitchen is getting hot. There's room for all of them but some will do better than others. Who makes the most desirable products and who adapts to changes in the automotive industry the best will decide who will prosper most.