If you rode through that fresh patch, your front wheel might have sunk in and turned abruptly, causing the fall.

I fell over last summer riding a loaded touring bike on a sand road in NJ blueberry country. I was only going about 5 mph and pedaling lightly. Front wheel hit a soft spot, sunk in and turned to the right. I came to a complete stop and fell over. Only thing injured was my pride, even though there was no one around to see it.