Younger dog being aggressive towards older dog

Recently our small dog who is a small terrier mix has been avery agressive towards our older lab mix usually out of nowhere. The terrier is about 2 years old while the lab is around 5.The terrier will attack the lab out of nowhere it seems usually biting at the neck and not letting go. Our lab is very good and doesnt fight back but gets very scared of the little dog. Like today for istance i just came home walked up the stairs and was greeted by the small dog i pet her and she went to run in a room while the big dog came out the little one just jumped on her neck and started biting her so i immediately tried pulling the little dog off but she held on very tightly and still wouldnt let go. im scared that the big dog will really get mad one of these times and attack back doing lots of damage b/c she is atleast 3-4 times the other dogs size. they are both female so what could be making the small dog act so aggressive?

Well the youngest one hasn't been sprayed yet so by doing so would that maybe help the problem

Maybe she wants all the attention. Or she wants to show dominance.

Spaying her will definitely help. Terriers tend to be very high intensity, domineering little critters, too. As Serena advised you, back your Lab's position. Make the Terrier wait until after the Lab has gone first at EVERYTHING.