There are a couple of things, but the most common is allowing the axle shaft to ride the inner lip of the seal during reassembly. If you let this happen, the shaft will nick or tear the lip of the seal, allowing fluid to leak as you have described.
Another cause is that you installed the seal improperly (doesn't sound like it). It could also be a bent axle shaft or bent axle housing (less common, but trucks that see heavy use have this problem from time to time). Check the housing for any nicks or gouges as well. When installing the seal, put a little multipurpose grease on the inner and outer lip if you aren't already - on the inner lip this helps if the axle does contact the seal on the way in and on the outer lips helps the seal seat and seal properly.
Also, check the o-ring that goes between the axle housing flange and the backing plate. If these deteriorate or tear you can get moisture and rust in the housing which will damage the seal.
Finally, the most common cause of recurring failures is a bad wheel bearing, which allows the axle to wobble a bit in the housing and damage the seal, allowing for the leak.
The bearing is cheap, but replacing it is a bear (need a press to do it properly, though there are other shadetree ways of getting it off and a new one back on).
