NXP Semiconductors is the fifth-largest non-memory semiconductor supplier in the world, and the primary provider of chips for the automotive market, which Qualcomm is considered to be eyeing in expectation of growth in the self-driving car market
Doubtful this is the reason, as despite what you read in the news, we in the semiconductor market do not expect the self-driving car market to ever be mainstream - that's what our automotive customers are telling us.
This is just part of the continuing consolidation of the semiconductor marketplace. With microcontrollers being sold at commodity pricing and so those margins getting thinner, the fastest way to expand while preserving margins is to acquire the competition. It's not about self-driving cars, it's about survival.