[yorkietalkjilly]

Dogs are no dummies - they definitely recognize pattern behavior and can easily put two and two together once they recognize one. In the past, you packed and then you left - easy as pie to presume that you will leave again next time you pack. The shaking could have been from worrying about your leaving them and getting insecure.

If you packed a suitcase and took them for a long, fun walk ending in a long play session and treats, and then you packed a suitcase again, some days later, they would probably get pretty excited, thinking another one of those fun outside play/walk/treat sessions was about to ensue in the future. Some dogs shake out of excitement and anticipation.

He could have been shaking from some noise or smell he didn't like outside and it was completely unrelated to the packing. Many dogs shake when they are ill or cold or want something really bad.

Did he stop shaking when you brought him back in the house? Has he done the shaking before when you packed a suitcase or during a walk?

If he stopped shaking when you brought him back in the house, you might start fiddling with the open suitcase and play-packing to see if he will start shaking again. If he does, to keep him from routinely establishing this type reaction to packing, you could just toss him some high-value treats or occasional squeaky toys to hunt and eat/play with from time to time as you pack, and/or, in between bouts of packing, just start a good game of tugowar, fetch or other positive redirection of his thoughts and energies or take them for a good outside walk and play session and return to pack some more. Tie packing a suitcase with something very positive and rewarding as well as leaving as a behavior pattern and, in time, he might start to think of packing as not all bad.