You could be experiencing the abs/4 matic because if the system senses a slip in the wheel it will apply and do what you are describing even its sunny. Conditions doesnt matter in this case. if senses a wheel slip for whatever reason the abs computer will send a signal to apply the brakes and engage abs.

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1986 300SDL, 211K,Dealership serviced its whole life
1991 190E 2.6(120k)
1983 300D(300k)
1977 300D(211k)