Too little or too much water, insufficient sunlight, moving the pot to a less sunny location, pests (by that I mean aphids or spider mites), pesticides, soil compaction, poor drainage, root bound plants.

PS - It is ok to water daily if it is very hot (think 97F for example) but, if temps are mil and the soil is moist to the touch (insert a finger into the soil to a depth of 2" or so), you can probably skip watering one day.

hibiscus do not like wet feet, make sure its well drained. Also - as the plant wakes up in the spring - it will usually shed any of last years leaves and grow new ones. be careful on the watering if all the leaves are yelllow - easy to overwater at this time. Another important thing to look for is spider mites - they hide on the undersides of the leave and suck them dry. They explode in the heat. if its really bad you see little webs and tiny mites running around. After any treatment, give it 2 weeks for the hibiscus to react and then you can follow up.