New Siamese biting older cat on the back of neck

We just recently Adopted a 1.5 Year old Siamese Female. Although she seems to get along great with our 15 year old Female; Just recently, while we were all snuggled up on the couch, she leaned over and bit the older cat on the back of the neck. She repeated this a few times, and although she didn't seem to bite down, whe can't understand why she does this?

Any Ideas?
Thanks
Heather

territorial or jealousy? Could be a number of reasons. The other day I had 2 of our cats in our bedroom. Fluffy, our 3 year old female, was at the top of the cat tree, and Tigger, our 10mo. old female, decided she wanted to play with Fluffy. They were batting at each other, & Fluffy decided she wanted to try and bite Tigger's head. She didn't really bite down, but nipped sort of...... werid behavior!

It could be affectionate or playful. If they don't act violent, don't worry about it.