That beats me. If I have to guess, I'd venture to say dealing with ambiguity is truly hard. As a production manager, you need to have unique insights on the market you're interested; you need to capture future trend; you need to understand human psyche, you need to convince your company to take risks and invest into something that does not necessarily have a future, you need to build trust with both the engineering team and the management, and you should be analytical enough produce supporting numbers. Many of such activities are ambiguous in nature, and getting clarity and buy-in out of such ambiguity is in high demand in the current market.
On a side node, engineers do sometimes need to function as PMs to build great product.