Morning Glories in a planter (depending on the planter size) located on a sunny deck can dry out quickly , so my thoughts tend towards the distinct possibility that the plants were not watered (while you were away on vacation) and consequently lost the lower leaves....additionally any sporadic rainstorms/wind may have dislodged and blown the leaf remains away...there may have been a certain amount of water reserve in the containers and plants but due to the sustained high temperatures any reserves may have been used up / exhausted...

The plant will likely still flower but be sure to water the plants everyday if necessary as the roots of plants growing in containers get heated in a way which does not occur if the roots are growing directly in the ground.

MG's will sometimes drop their lower leaves as the plants mature and move nutrients to the areas of most active growth and the dropping off of the lower leaves is exacerbated by water (and nutrient) deprivation.
There is a limited amount of nutrients stored in the leaves and stems but fresh nutrients require water to dissolve additional nutrients in the growing medium and facilitate absorption into the plant roots.

I am fully aware that water deprivation can be used as a means of forcing some plants to flower but this is usually not required for most MG's and results in plants which are more susceptible to disease and any seeds produced will definitely be of less than optimum quality.

The show quality asagao plants in Japan always retain all of their leaves , are usually grown in very small containers , watered daily and receive nutritional supplementation with special formulas that are many hundreds of years old...so although there are people who manage to grow some common MG species (in the ground) by neglecting them , the best show quality plants receive mucho attention and the results are beyond the ken of most Western Gardeners...

Hope my thoughts may prove to be useful...(if not in the short run , then perhaps in the long run)

regards,

Ron