I can't say I have seen one seize either, as an alternator usually fails due to brush wear or some other electrical event. Tightening the belt too much can overload the bearings, and begin the process, but the brushes usually can't handle the extra movement of the shaft as the bearings wear.
If your mechanic said it was about to lock up, the problem was well along, and it is feasible you might have heard some extra noise from the alternator. Then it is also feasible that whatever caused the problem, if it was not the belt being overtight, just happened and you found it. Other causes might have been getting water in the bearing (flood, or going into a deep puddle, or worst of all, driving on the beach and getting saltwater in there), or developing an electrical problem on the rotor that passed current throught the bearings. You never said why you took the car in and had the alternator examined and ultimately replaced though, so this speculation is kind of ungrounded and likely not of value.
1986 Euro 190E 2.3-16 (291,000 miles),
1998 E300D TurboDiesel, 231,000 miles -purchased with 45,000,
1988 300E 5-speed 252,000 miles,
1983 240D 4-speed, purchased w/136,000, now with 222,000 miles.
2009 ML320CDI Bluetec, 89,000 miles
1971 220D (250,000 miles plus, sold to father-in-law),
1975 240D (245,000 miles - died of body rot),
1991 350SD (176,560 miles, weakest Benz I have owned),
1999 C230 Sport (45,400 miles),
1982 240D (321,000 miles, put to sleep)