DB file sequential read

Hi,We have a simple program the code is given below. It simply truncates and then inserts a table. It was working fine. Only since the last few days the program started taking huge amount of time. We observed it is doing too much of db file sequential reads. There was no change in the code anywhere during this time. Any idea on how this can happen?

truncate table xxia.xxrpt_ship_carton_hist_s;

INSERT      /*+ APPEND */INTO xxia.xxrpt_ship_carton_hist_s
   SELECT *
     FROM xxeul.carton_hist_shipped;

Also the program runs fine in dev and test instance but hangs in prod.