Livingdead,
Perhaps you'd like to read the Boeing QRH for the Boeing 737.
I quote:
"Traffic Aviodance
Immediately accomplish the following recall whenever a TCAS traffic advisory (TA) or resolution advisory (RA) occurs.
<snip>
For TA:
Look for traffic using traffic display as a guide. Call out any conflicting traffic.
If traffic is sighted, maneuver as required,
For RA:
<snip>."
Boeing QRH certainly differs to you knowledge of TCAS.
Our ops manual backs this up too.
So just maybe the said Captain saw something he didn't like and therefore maneuvered as appropriate?