Originally posted by: Vibhishna
It was different in different versions. This may be a repeat of my previous post, but I'll explain again anyway.
In the Valmiki Ramayana, Ravan siezed or grabbed her when she had crossed the Lakshman Rekha.
In the Tulsidas Ramayan, sometime before this incident happened, when Lakshman had gone to collect fruits, roots and other food form plants, Ram had told Sita that he was going to play a more human part and til he calls her to remain in Fire's Abode. Immediately, Sita Devi, with the image of Lord Ram's feet installed firmley in her heart entered the fire and left behind a shadow of herself - no less in beauty and charm. Even Lakshman did not know what Lord Ram had done.
In the Kamba Ramayan - Since Sita Devi was a staunch Pativrata, Ravan did not dare touch her. Hence, he tore apart the land on which she lived - the cottage and all and flew off and set down this enitre settlement in the Ashok Vatika. He didn't need too much time to dig up the earth. He could do it in an instant (was too strong)