How does the bonus system work?
From what I’ve seen, it depends on the status of the meta event. If many events contributing to it (e.g. upgrading/defending outposts in Verdant Brink) finish quickly after one another you’ll quickly get many chests in a row. If only a few events finish, the meta chests will come in slower.
The goal seems to be to get the map-wide meta event to finish tier 3 before the end of the event cycle (e.g. end of day/night in Verdant Brink), since that is the point where you will get the final (and most valuable) chest of all.
Hover over your mini map while in any of the new HoT maps. You will see two bars above your minimap appear. The top bar is map progress. There are 3 tiers to each map, with 4(?) blips for each tier. Every time your map does enough events the bar moves to the next blip and you receive a chest for each blip you pass. As you reach higher tiers the rewards become better.
The second bar, underneath the first, is your personal participation. The higher this bar goes the better your personal rewards. It can reach a maximum of 200%, which means you get twice the “normal” rewards for each bonus blip you reach.
Hope that helps!