Bans are manually implemented by Bungie staff after confirming a violation, so they can happen at any time, but are generally done in batches. It's not unusual to see different people get banned within days of each other and for different reasons.
For a list of reasons why someone can be banned, [url=https://help.bungie.net/hc/en-us/articles/360049517431]see here[/url].
Some of the most common reasons people get banned are due to violating one or more terms of service: running a cheat program while playing the game (regardless if it's intended for D2), network manipulation, doing an account recovery (free or paid), or playing in a fireteam with someone who was cheating.
This doesn't make any sense. If this is true then I would be banned along with everyone in the Control lobby from LFG discord because we all played with someone who was banned.
Also, I know for a FACT that not all bans are manually implemented. If someone runs a cheat program and it's detected by battle eye you're telling me that battle eye won't instantly ban that player? That a Bungie employee has to manually review it and ban them?
In fact, this is a quote from DMG himself. "Destiny 2 is adding "automatic bans for a variety of cheats" in time for the first Trials of Osiris weekend of the ongoing Season of the Lost."
So which is it, manually or automatic?
I'm going to keep this post up to date and see if any of these three people get a ban appeal. It would be tragic and look incredibly stupid for three innocent people to all be banned on the same day.
There are also automatic bans that are applied by BattleEye, on Steam, but that's specifically against cheats and other programs that attempt to modify the game's code that BattleEye can detect. There are things that don't get caught by BattleEye, and there are much more players on console than PC, as well as many other reasons people can be banned that require staff investigation.
If you were in a lobby with someone who was banned in that moment, that does not necessarily mean they were banned for using a cheat, nor does it mean the time you were playing with them is when their violation occurred. If it was a manual ban, then they did something days, possibly weeks prior.