Table lock overflow

Hi,

During the performance monitoring, we observed lock table overflow occurred for T-Code VT01N(Shipment creation) in production system on 28-MAy-08 at 15:30:10 system time for user ID XXXXXXX. In the past two months, lock table overflow occurred twice, once on 11-Apr-08 for user ID YYYYYY and on 28-MAy-08 for user ID XXXXXXXX, both for T-Code VT01N.

Can anyone please help me by telling the reasons and the corrective measures need to take to prevent this issue.