Hello Beggingpardon, There are many reasons for this to happen, but let me assure you, all leaves will eventually go yellow and eventually drop off. It is how many at once and when their should be no reason for leaf drop, that you should worry. Below is a cut and paste that covers some of the logic behind leaf drop.

Yellow Leaves
& Leaf Drop
Hibiscus rosa-sinensis 'cultivars' are classified as an evergreen & as such, normally do not shed all of their leaves in winter.
There are environmental influences that will cause the shedding of some or most of the mature leaves.
This is termed a natural cycle of the plants development, where the nutrients are transferred to the growing tips while the old yellow leaves fall to the ground to increase the mulch & the nutrient content of the top soil of which the surface feeding roots of Hibiscus thrive within.
The yellowing of leaves can be triggered by pruning, feeding, sudden change of soil moisture content & higher than average temperatures & incorrect pesticide chemicals or concentrations.
The total yellowing & loss of all leaves may indicate a root problem.

Even this info does not detail the many circumstances that can cause yellowing leaf drop. If you are not able to see the answer here above, write back and I will detail further reasons.

Regards, Brian Kerr.