don't understand this call

The runner doesn't have to slide, but if he does slide it must be legal. Sounds like the umpire ruled the runner did something illegal (probably the contact after the runner hit the base).

Interfernce on this play, if judged to be to "intentionally" break up a double play, then yes the penalty is both runners are out. Sounds like what ya got.

The FPSR is very misunderstood by many, umps and coaches alike. If they called a FPSR then they were incorrect unless they ruled the fielder was behind the bag when contacted. If the fielder is on the bag or in front of it he can be slid into legally. The other thing they may have thought was the slide was illegal. This could be foot above the knee, pop-up slide or rolling slide. If this were true then they should have also gotten an ejection. Unfortunately there are many variables that could have happened including they just didn't know the rule and blew it. The only way to know for sure is to ask them what they actually called.