You answered your own question...you have "established" guppies.
General rule of thumb:
Guppies are notorious for not being kind to each other. It is usually suggested 3 females per 1 male guppy. Males are very active breeders, if you only have 1 female to 4 males...you just gave her a death sentence.
Also are you aware, the opposite side of the coin....if you have 1 female who is pregnant, she can give birth to young every month...even without a male. Females can store sperm up to a year and fertilize their own eggs.
Also a 10 gallon tank does not give fish any room for hiding.