In plain words
What was the reason for this? No one has explained it properly.
Kausalyā and Kaikeyī took their bowls of pāyasa happily and went to their shrine rooms. Kausalyā hoped that, since she was the chief queen, the son born to her would be crowned. Kaikeyī was happy, thinking that by Daśaratha's promise to the king of Kekaya her own son-to-be would be crowned. But Sumitrā knew that for the son to be born to her there was no such chance. She bathed her head and went up to the terrace to dry her hair. There were no fans or hair dryers in those days, after all! She set her bowl of pāyasa down on the terrace, and while she was drying her hair a kite came from somewhere and snatched the bowl away. ‘Alas, the bowl of pāyasa is gone! Who knows whether my lord will be angry!’ Grieving so, she ran downstairs. Consoling her, Kausalyā and Kaikeyī said, “Don't grieve at all; we will give you part of our pāyasa.” Kausalyā poured half of her pāyasa into a golden bowl. Kaikeyī too poured half of hers into it and handed it to Sumitrā. In those days even co-wives lived together in great love, with no differences of any kind. The three took their bowls of pāyasa, went to Daśaratha and bowed. Vasiṣṭha too came and gave his blessing, and they drank the pāyasa. Then all three conceived.
To Kausalyā Rāma was born. To Kaikeyī Bharata was born. To Sumitrā Lakṣmaṇa and Śatrughna were born. From the day they were born, Lakṣmaṇa and Śatrughna would neither drink milk nor sleep. Nothing but crying, crying, crying! Neither medicines nor mantras, yantras and tantras did any good. Then Sumitrā went to Vasiṣṭha and, in great distress, told him of her children's state. He sat a while in meditation: “Mother, do not grieve at all. Lay your elder son in Rāma's cradle; lay your second son in Bharata's cradle,” he advised.