Why do Catholics "go to rosaries" when someone dies?
The short answer
When a Catholic dies, friends and family often gather to pray the rosary together for the repose of the person's soul. "Going to a rosary" is simply shorthand for joining that gathering.
Praying for the deceased
After a Catholic dies, it is common for friends and relatives to come together and pray the rosary for the repose of the person's soul.
The expression
Saying you are "going to a rosary" is a shorthand way of saying you are gathering with others to pray the rosary together.