alt+c and alt+shift+c cycles tabs - How to disable?

[yogapotato]

When I enter alt+c, it goes one tab to left, and alt+shift+c goes one tab to the right. However, I can't find this option anywhere in the command listing, and when I try to unmap "c", it tells me the key is not mapped. Is this behaviour intended?

[gdh1995]

unmap <a-c>
unmap <a-s-c>