Sigma (and other 3rd party lens makers) must reverse-engineer Canon's AF compatibility. At some point, the cumulative changes preclude the ability of older lenses to work with new bodies - some old Sigma lenses can be re-chipped to work with new bodies, but only back so far (usually within their long warranty period).
Canon does make sure their old lenses are compatible, but even then, sometimes they fail. In fact, the example you list is one of the 'failures'. You only thought the 35-80 worked fine on your 60D.
Ok, maybe it did from a practical standpoint, but while your 60D has 9 cross type AF points, with the 35-80 lenses only the center one acts as cross-type, the outer 8 points are just lines. Canon issued a Product Advisory
about that problem. Actually, it's less of a problem for Canon since not many people use those old lenses, but more of a problem for Tamron, because several of their newer lenses 'borrow' the lens codes from those older lenses to spoof the AF system, and so fall victim to the same problem.
That was probably more than you wanted to know...