RDGO1, Let me congratulate you first on your gc approval.
There are many reasons why your wife's case (is your wife dependent?) was transferred:
1) medical results are not clear or unfinished
2) fingerprint results are not clear
3) misleading information on background history
4) out of status in the past in USA
5) criminal history (shoplifting, several traffice citiations, etc.)
6) security check names are not clear
7) birth certificate is not clear or no enough sufficient
8) need more information from your wife or you like missing documents
9) officers needs to verify any evidence to prove that you and your wife are married (show apt lease agreement, deeds, photos, joint names at bank, both driver license displaying same address to prove that you and your wife live together, etc.)
10) missing visa status (H4 approval notice, or F4 stamp, I-20, etc)
11) I-94 status (maybe you didn't submit or etc.)
12) financial support to demonstrate that you have ability to support your wife (paystubs, tax returns, etc.)
13) random checking
14) if your wife was on B2 (tourist visa) and married you, they need to know why you married her and you must explain or show that marriage is in good faith (officer would think the marriage is fraud).
14) so many reasons..
Your wife will be fine if her case is straightfoward. It will be just formality.
Congratulations on your gc approval and Good luck to your wife. Don't worry. she will be approved.