My 2 lab/collie crosses are almost 8 and a half. I rehomed them 6 years ago and TBH it's been plain sailing. However, in the last few months they've been a nightmare at the end of their walk when I'm at the car. They are guarding it, trying to see off anyone or any dogs who come anywhere near the car. I have them on the lead still but they are pretty strong and I'm worried they'll pull out of my hands. They growl and bark - generally very aggressive sounding. The problem is mostly at weekends when I walk later and see more dogs/people. During the week I walk at 5am, see the same people and they're fine.
Why are they doing this suddenly now and what can I do to stop it?