Does anyone have any theories on the recent $10m gold coin find in Saddle Ridge?

I don't know what you are looking for in terms of "theories". And all we have to go on is the information in the press releases. However there were plenty of useful pieces of information revealed by the press:

The coins were buried in eight containers.
The hoard contained 1,427 coins - with a face value of $27,980.
The dates range from 1847 to 1894 (although a distribution was not given).
Each container had coins from a similar time period. (1840s-1850s in the earliest tin, etc.)
The coins were nearly all in mint or near mint condition, indicating they were collected near the time of minting, then kept.

The last three indicate the hoard was collected, and possibly interred, over the course of nearly 50 years. Some people had suggested the gold was from a bank or stage coach robbery - a very unlikely scenario.

The dates on the coins correspond to the timing of the California gold rush, which occurred in the region where the hoard was found. Although the rush itself is considered to be the period between 1848-1855, significant amounts of gold were still being extracted up through the 1880s using sophisticated hydraulic techniques.

The news articles did not indicate how the coins were distributed over the time frame, but once they go up for auction we will have that information. If we [poorly] assume that the coins are evenly distributed over the time period, it works out to about $600 per year. Considering common (non-farm) laborers earned $300-$500 per year during that period, I would expect the hoard represents the savings of someone earning 3-6 times the wages of a laborer - solidly upper middle class, but probably not considered uber-wealthy. Perhaps a successful merchant or professional.

It has been shown by Karen Clay, Randall Jones, and Malcolm Rohrbough that merchants made far more money off the gold rush than the prospectors ever did. This would also point to the hoarder probably being a merchant of some sort - perhaps a store owner, or a hotel or saloon owner, perhaps even a prostitute.

Why was the hoard buried? European hoards were often buried to avoid capture by invading forces. I don't think that was the case in Sierra Nevada. Before the Banking Act of 1933, financial panics happened every 15-20 years, bank failures were common, and depositors' funds were not insured. Around the time the hoard was accumulated, there were financial panics (followed by depressions) in 1837-1843, 1857-1859, 1873-1878, and 1893-1897. Keeping ones life savings in a bank for decades might not have been the safest place for the money.

Why did it remain buried? At this point, I go into almost pure speculation. I suspect the hoarder may not have had a large family, which would have required significant financial resources to raise in an upper class life-style. If so, the hoarded gold would likely have been distributed as inheritance and/or dowry. Considering the gold was collected over nearly 50 years, I suspect the hoarder died of age or sudden illness without telling any friends or family the location of the hoard.