Comment by BinaryIgor

Also worth mentioning - it happens more often when you set journal_mode=WAL, which is not a default.

The default is DELETE mode, where the rollback journal is deleted at the conclusion of each transaction. What's more - in this mode (not-WAL), readers can coexist, but they do block the writer (which is always one) and the writer block readers - concurrency is highly limited.

In WAL mode - which pretty much always you should set - there's also at most one writer, but writer can coexist with readers.