Why did Michael Nunan leave Sturt?

Found this on Google Books - "1978 South Australian Football Season Report" by Trevor Gyss.
https://books.google.com.au/books?id=0bWaBgAAQBAJ&pg=PP1&dq=1978+South+Australian+Football+Season+ReportBy+Trevor+Gyss&hl=en&sa=X&ved=0ahUKEwjJ-IS4kd_KAhUJmpQKHRqaAekQ6AEIHTAA#v=onepage&q=1978%20South%20Australian%20Football%20Season%20ReportBy%20Trevor%20Gyss&f=false
Unfortunately, the preview doesn't let you peruse the whole book, but there are some good photos within.
There's some discussion about pre-season movements between clubs, and it notes on page 9 that Michael Nunan was picked up by Norwood after traveling overseas and "being discarded by Sturt". I didn't know that this was the case - can anyone shed light on why he was discarded? I recall he was a key part of Sturt's dominant ruck set-up, certainly until 1976, and assumed he left for Norwood on his own accord.
https://books.google.com.au/books?id=0bWaBgAAQBAJ&pg=PP1&dq=1978+South+Australian+Football+Season+ReportBy+Trevor+Gyss&hl=en&sa=X&ved=0ahUKEwjJ-IS4kd_KAhUJmpQKHRqaAekQ6AEIHTAA#v=onepage&q=1978%20South%20Australian%20Football%20Season%20ReportBy%20Trevor%20Gyss&f=false
Unfortunately, the preview doesn't let you peruse the whole book, but there are some good photos within.
There's some discussion about pre-season movements between clubs, and it notes on page 9 that Michael Nunan was picked up by Norwood after traveling overseas and "being discarded by Sturt". I didn't know that this was the case - can anyone shed light on why he was discarded? I recall he was a key part of Sturt's dominant ruck set-up, certainly until 1976, and assumed he left for Norwood on his own accord.