Currently, in V3.5 this game feature was eliminated
After finishing a Base raid there is a chance for pirates to appear (~every 50-75 battles). Pirates have the same ELO as your Fleet Raid, so they might wipe your fleet because you're out of position after a Base raid. When encountering them you have the option to Flee with a random % chance of success, or you fight them and get one of the following rewards:
When refreshing your browser while pirates appear your WHOLE fleet will get wiped.
Please note: Pirate fleets are actually other players fleets