Transaction (process ID) was deadlocked on lock resources with another process and has been chose

Recently, when processing data in a table in SQL Server 146 using multiple threads, the [transaction (process ID) was deadlocked on lock resources with another process and has been
Chosen as the deadlock victim. Rerun the transaction error is displayed. I thought it was correct.ProgramThe reason why the lock is applied causes a deadlock. Later I found that this is not the case. The table I am processing has not set a primary key, and the update is based on the primary key.

I think about it again because I updated this table through the SQL server2008 built-in import table function. The original table has a primary key, however, the new table imported does not have a primary key. This problem is unclear. If you know it in advance, you will not have this problem if you set the primary key.