It was done on purpose.

It is the gondola the other spoke to." Their assailant was turning also, and in a few seconds was in pursuit.
Francis understood it now.

The gondola they had been following had noticed them, and had informed their friends, waiting off Saint Mark's, of the fact.

Intent upon watching the receding boat, he had paid no further attention to the four-oared craft, which had made a turn, and lay waiting in readiness to run them down, should they follow in the track of the other boat.
Francis soon saw that the craft behind them was a fast one, and rowed by men who were first-rate gondoliers.

Fast as his own boat was flying through the water, the other gained upon them steadily.