This is the 11147th most frequent Turkish word.
duyunca
"When heard" or "upon hearing."
Here, 'duyunca' means 'when I heard', connecting the action of hearing with the resultant feeling of surprise.
Haberleri duyunca çok şaşırdım.
I was very surprised when I heard the news.
In this usage, 'duyunca' translates as 'when she/he heard', linking hearing about the incident to the emotional reaction.
O anı duyunca gözleri doldu.
When she/he heard about the incident, her/his eyes filled with tears.
Here, 'duyunca' conveys 'upon hearing', showing the immediacy of the emotional response to hearing the news.