Jim Allenby left Glamorgan because he was disappointed not to be offered the captaincy at the Welsh county, according to Somerset director of cricket Matthew Maynard.
All-rounder Allenby, 32, announced he was joining Somerset in November.
Former Glamorgan coach Maynard seized on the opportunity to be reunited with Allenby.
"I know he was disappointed not to be offered the captaincy at Glamorgan," said Maynard.
"He led them in the T20 and almost got them to finals' day. Just one crazy over up at Lancashire changed that.
"I think he just wanted to lead and was disappointed that he was not given the opportunity."
|Jim Allenby's first-class career|
|Batting average: 40.83|
|Bowling average: 25.79|
Born in Perth, Australia, Allenby began his first-class career with Leicestershire before joining Glamorgan in 2009.
He enjoyed an excellent 2014 season and was named Glamorgan's T20 player of the year after leading them to the quarter-finals.
The former Western Australia player was also their leading Count Championship run-scorer in 2013, and signed a new four-year deal last summer only to announce his departure months later.
Allenby, who is eligible to play for England, has 6,656 runs and 254 first-class wickets to his name.