It was could have been the number of eggs she had availible, or the male sperm could have not been great. Males sperm recycles every 30-60(I think I cant remember exact days). If they bred with he was at the end of the cycle, there may not have been enough good sperm for a good litter.
Unfortunatly there is not any one anwser. the cause could be anything you think of, and everything you don't. My mini aussie bitch only had 4 last litter, and she came from a litter of 9. Nature is what it is.
How many time did you breed your female. and did you give the male a rest in between ties? Some males actually need a little help in the tie department,with out getting too graphic, that maybe the problem. Good luck with those cute babies.
Such a huge array of things it could be. Egg production, sperm count, timing, tie length, time between breedings (not only your female but if any others were bred as well inbetween),health...not just a regular health check, but uterine scrapings, blood counts, sperm count, etc.
A ton of other things too, that I cant even think of can cause it.