i used to have a terrier mix spitz n no wi have a terrier mix shih tzu. realised that both of them get excited quite easily n sometimes even hyperactive. wonder if its got anything to do with having terrier blood.
probably bored. all dogs - no matter what breed - if bored will have excitable natures. try exercising them more or give them toys to play with to keep them busy.
"The greatness of a nation can be judged by the way its animals are treated" Mahatma Ghandi.
Terriers are known to have a very high prey instinct.I think that it is because the most terriers are breeded as hunting dogs.I would not call it hyperactive.i think you need to let them run more and go for long walks with them and give them the opportunity to use their energy.