Investigating a deadly fall

When we moved to our new home, we rented a truss, high up under the top eaves, to a working couple of robin. After building their nest in pretty short order, they laid some eggs and soon thereafter had a couple of chicks.

For the last weeks, we'd go in the south patio after lunch to sip our coffee, get some warmth from the sun and would observe the feeding routine of these two youngsters by their parents.

The latter would generally take turn to feed their young and would seldom leave the nest unattended. Yesterday we observed that at time, the nest was left woefully on its own for long periods of time.

This kind of worried my wife. I didn't particularly paid attention to the situation, until, an hour or so later, I saw a dead baby bird that had crashed to its death on the cement patio, 28 feet below the nest.

Did the youngster ventured out of the nest, lost its footing and dropped, was it pushed by its sibling or was it a case of infanticide? I'm investigating and plan to interview the parents, the other robins, magpies and falcons that roam the area...