I look at failed bearings daily for a bearing manufacturer. Usually bearing fail with the onset of spalling (for any of a number of reasons), then progress slowly until it gets louder and eventually reaches complete failure. I can't conceive of (nor have I seen) a failure where a bearing goes from good to seizure on the freeway in under a minute without prior warning.

You hint that you are unsure of the bearing preload and regreasing procedure you used at the last service. WHy were the bearings retightened? Normally they shouldn't be touched unless they are either regreased, replaced, or opened for inspection. I'd like to point out that most bearing failures result from improper installation, and not from some intrinsic defect in the part.

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95 E320 Cabriolet, 159K