My background is in the use of PROC MIXED, and in that context, I have read warnings against using Information Criteria to compare models unless the exact same observations are included in each.
My best guess is that there are missing values in the covariates and that SAS drops all observations with a missing value on a covariate even to fit the Intercept Only model. To me, this makes sense because the 2 models cannot be compared unless they are fit on the exact same observations. When you fit a different model, different observations are dropped because the included covariates have a different pattern of missing data.
During a quick check, I could not find this issue addressed in the SAS Help or on the Support site so take this opinion with a grain of salt. Hopefully someone more knowledgeable will chime in.