I had the same problem couple of weeks ago, Lets Analyze my situation when I was loading the data, steps are as follows:

1.disable constraints
2.truncate table
3.load data into table

In 2nd step (say Session S1) DDL statements ALTER,DROP and TRUNCATE statements locks the table.
3rd step (say Session S2) locks the table for insert.

Hence the ALTER statement in S1 is blocked by S2 lock. Until S2 commits its INSERT and release the lock S1 can't complete its task.

so by mentioning commit=99999 (say no. of rows) the problem will be resolved.