That is hard to say without looking at a trace of the session or a statspack report. It could be that the application is doing an 'insert into table_a select ... from table_b'. If auditing is enabled, depending on the level of auditing, that can cause extra I/O as well.
The best way to track this down is to enable a trace on the session in question. The trace will capture any and all SQL that is run during that session. Then use the tkprof utility to format the trace file generated from your session so that it is easier to read. From there you should be able to identify what is causing the problem.
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