1st: because he played very well in his time there.
2nd: Because Van der Wiel will probably leave (75% certain), maybe also Vertonghen (50/50) will leave and Emanuelson (75% certain), that makes 3 defenders that might leave, they will be in need of a few defenders and Grygera with his past there, might be interested in returning.
3rd: They have financial problems so if they can get him in exchange, togheter with some good money, they will be interested in letting some players go.