Assuming that the app server is running on a dedicated server I would first check the memory available on the machine and the memory configured for the app server. AIX will make use of the available memory before using the page space. In your case you should look whether the physical memory usage is high or not. If yes then you should check whether the system is correctly sized. In addition to that check the paging space configuration based on the note 1121904 - SAP on AIX: Recommendations for paging space also check if AIX is set up as recommended
973227 - AIX: Virtual Memory Management Tuning Recommendations
1972803 - SAP on AIX: Recommendations
If the issue is happening only on one of the application servers then check if the number of users using sent to that app server is huge. Also check if all the AIX servers are on the same patch level.
Without checking the system it is difficult to understand where the issue is. Start with the memory allocation and usage, system configuration like ulimits, AIX parameters etc.