I work for an electric company and I can shed some light on some of your points.
tl;dr: your electric utility isn't turning down your power intentionally but they may be experiencing heavy load that your circuit can't handle well.
Why would the grid operator reduce voltage at night?
It is highly unlikely (read: generally impossible) that distribution company is turning down the voltage. Electric companies don't have dials to turn these things down, because that is complicated and expensive and it's not very useful.
Your other conjecture, that perhaps something is loading the system, is much more likely. (More in a bit.)
As a side note, you (as an ordinary home customer) get your power from your local electric utility company not a grid operator. Southern California Edison also operates transmission and generation, but the energy market, especially in California, is complex. Either way, you're a utility customer not a grid customer. This matters because why electric utilities and grid utilities can (and will) do for, and to, their customers, is very different, specifically on the points that you ask about here.
Does it save energy or something?
Your thought on this is correct. It does not save energy, power, wear, or anything like that. This is why it's not generally done.
One alternative explanation I can think of is that some nearby large industrial customer starts up their machines at 10pm. That doesn't sound right in my residential neighborhood with no heavy industry nearby.
This is quite possibly very close to the reason. You may have the right idea but you may be envisioning the wrong details.
Other than factories, other similar causes, with some relevancy notes:
- Crypto miners and data centers. Data centers are generally known to the distributor so they're less likely to have a hard cut. Crypto miners tend to pop up in places where power is available (a residential neighborhood may be a great place) and are very capable of suddenly unleashing very high demand on schedule.
- EV charging. I will mostly defer to @SpheroPefhany about this. It may even be not you or your neighbor, but reduced rates at charging pylons in the neighborhood.
- Less-industrial uses, like water treatment. There are a great many services like this that could potentially run at night for a number of reasons. This could also imply a change in the nature of the load (power factor). That is, there may be a business that uses a lot of (for example) heavy motors during the day, for which your electric company compensates. At night, when they turn off this load, the compensation behaves as overcompensation.
It is likely that this is not something that your power company is doing but rather something that is done to them. Assuming they're competent (a good assumption because you and very, very many other people in Southern California almost always have workable power), they probably would prefer not to have this happen at 10pm daily. On the short term, they most likely do not have the resources to fix it.
California, and especially Southern California with regard to residential consumption, is a very difficult market for an electric utility. Much of the state (republic, I believe y'all call yourselves) is chronically tight on energy supply for a number of reasons. Anything financial is always a challenge and upgrading a part of a distribution network can be expensive. More remote neighborhoods or less-new developments are even more challenging.