JOIN can if it is not done on the right matching fields turn into a multiplication of the rows of the two tables involved. This is called a cartesian join/cartesian product.
So if you do a join with no common fields (common field names) then you get m x n rows as a result. Every row from the first table matched with every row of the second table. If the first is 10,000 rows and the second is 100,000 rows you will get 1000,000,000 rows unless you run out of memory (or time) ....