This comment on HN sums it up nicely:

1. They Osborne'd their product line by prematurely showing off new products that weren't ready.

2. They tried to meet the iPad head-on in the consumer market by calling their device a Playbook but their strength is in enterprise. Thus the marketing message came out confused and having a pompous “Amateur hour is over” campaign sealed their own fate.

3. Repeated, worldwide BBM network failures that went on for extended periods of time, alienating its most loyal customers when RIMM needed them the most. This was their core competency and they neglected it. Having alternatives on competing phones like iMessage, WhatsApp etc didn't help.