Simply stated, he committed an unprecedented Osborne Effect by killing one brand that was still selling (yet not growing in market share which in itself the market was growing) and then coming out with an unpopular mobile OS that meant Nokia had an uphill battle whereas it truly could have been avoided.
BB is a different story, but with similar ingredients. Bad management, bad decisions, bad delays and a misunderstanding of how the market was going to change. Those are the similar ingredients. BB thought their enterprise inroads would keep them ahead of most; it didn't. BB also thought that nobody wanted a touch screen; they were 100% wrong there. Nokia disregarded touch, but built it anyway. There's some other differences... but BB was just plain arrogant whereas Nokia was just plain disillusioned.