DSS staff know that some people suspected of benefit fraud will tell them a pack of lies and persuade someone else to back up their story. (I'm not suggesting that applies to you and your partner, but the DSS staff have to regard everyone with the same degree of suspicion).
If the person who backs up the false statements of the benefit claimant is later found to have lied, both people can be charged with 'conspiracy to pervert the course of justice'. It's because of the possibility of you facing such a charge (if you lie) that the DSS question you under caution, in order that your statements can be used as evidence against you in court.
Chris