What you're observing is a phenomenon known as the fastest gun in the west. Folks feel the rush of knowing that they can answer a question, and try to get an answer written as quickly as possible. Being first does have certain advantages when it comes to maximizing the points that you'd earn from any given answer.
However, don't be too quick to attribute the series of refinements that they put into their answer as being anything sneaky or underhanded. They may well have known something, and realized that they forgot to include it. Or, perhaps, they simply find additional inspiration in other people's answers, then go back to make their own even better. Many will do this quite transparently, e.g.
And as General Zod noted in his answer, some universes just don't deserve to live.
What gets a little fishy is when there's strong evidence of another user incorporating too much of someone else's work into their answer and then passing it off as their own. Still, take care here - some simple questions often result in users posting and tweaking basically the same answer completely oblivious to one another.
Remember also that each answer should be self contained, so you might also see answers that summarize several others, and then go on to explain something else that all of them completely missed.
As long as it's obviously in the spirit of people producing the best possible quality that they can, it's generally benign and an artifact of the motivation that highly competitive environments can bring.
If it looks fishy despite that, get a moderator involved.