Since DSTV operates roughly in the 11-12Ghz range it's not supposed to happen as normal FTA is in the range of a few hundred MHz. However, 2 theories may be possible:
Harmonics...those who remember this from physics class - though I find it unlikely as it would change every time the DSTV client changes channels hence requiring a retune on your telly - channels from the transponder do differ by a few MHz
The RF cable from the decoder to the TV could be acting as an aerial - either if the RF modulator on the decoder was of low quality, or the TV tuner has some sort of backfeed (low quality too). Since you say this used to happen way back, my money's on the former...as what has changed is the decoder models. The coax type used then could also be suspect as it probably didn't have proper shielding. Many people still operate old tellys but I don't see my neighbours' channels when I do an analogue scan. Someone with a decoder from waaay back (when having DSTV was swag) should be able to confirm this :)