Thanks for your kind suggestions and sorry for the late update.
Actually, TABLESPACE was created with PRIQTY and SECQTY values as -1 which allows DB2 to increase the primary or secondary allocations dynamically when data grows but somehow it was not happening to my tablespace.
I have dropped and recreated the tablespace. Now, DB2 is able to manage the primary and secondary allocations efficiently.
Not sure about reason why it was not happening earlier and happening after recreation