Why did my guppy die?

see last vweek my 5 month old guppy died he had 1 cat fish 8 neons tetra 3 guppys and i want to know why it died ho and also just before it died its female guppy bacame pregnant so why did it die i fed all the fish as the pet store guy told me it wase in a18 22 gallon tank so why did it die