I did a survey and asked the above question.  Here are some of the answers I received:

• Hired as the CAD manager then implemented  BIM
• Hired after doing BIM consulting and training
• Happened to be the most BIM savvy at the firm at the time. Was an architect who helped implement Revit in a firm and then looked for a firm to do BIM Management full-time.
• Started as a CAD Manager and moved more into BIM
• I am a Recovering Architect who moved into CAD support due to me being good with technology (and an admittedly crappy architect). Learned as much as possible and moved up the ranks.
• Directly applied after seeing it on a job board and also sent a copy of my app to a person who works at the firm (through a network contact)
• Advanced through projects using Revit, firm noticed abilities and moved me up.
• Migrate from CAD Manager
• Headhunted
• Previously the IT/CAD Manager in our firm. Took charge of exploring the option of going to BIM and I am the most technically savvy person in our office. The role just fell on my shoulders.
• Purely by chance and having experience in the software.

After reviewing the more than 60 answers, I came up with the following:

Summary of how most people come to be BIM Managers

• Most grew into it from within the firm
• Some hired into it from outside
• Some create the position and fill it
• Some were forced into it – or took it on by default
• Some were already doing CAD Support and you expanded to BIM
• Most achieved the title via software skills and knowledge