I have grown onions for years. Last year and now this year too, I bought sets at the store instead of starting from seed inside (cats eat the seedlings). Last year the yellow ones were awesome but the red ones all got flower stalks that greatly reduced the size of the bulb! The garden center had not had anyone else complain of it. This year some of the red ones are doing it again. I am keeping them trimmed off as much as possible. I would like to know why this happens and what is the best way to get nice big red onions as well as the yellow.

Extremely cold weather during early season growth can condition onions to bolt and flower. Also a period of warm weather followed by cold weather which makes the onions go dormant and then more warm weather with growth beginning again can trick onions into thinking they have gone through a winter. Being biennial, it is natural for them to set seed the second year.
Onion sets that are larger than a dime are also more likely to bolt and send up flower stalks.