I'm getting the following error in a Process Builder rule, and I'm not sure why. The rule fires when a user lookup field on the Account changes. When that field changes, it changes all of the Account's Contacts ownerId to equal the new field value.

Here's the error:

An error occurred at element myRule_1_A1 (FlowRecordUpdate).
    UPDATE --- UPDATE FAILED --- ERRORS : (ALL_OR_NONE_OPERATION_ROLLED_BACK) Record rolled back because not all records were valid and the request was using AllOrNone header --- for SFDC record with ID : 003A000001VHDehIAH, 

    This report lists the elements that the flow interview executed. The report is a beta feature.
    We welcome your feedback on IdeaExchange.

    Flow Details
    Flow Name: Update_Contact_to_Pipeline_Rep
    Type: Workflow
    Version: 1
    Status: Active

    Flow Interview Details
    Interview Label: Update_Contact_to_Pipeline_Rep-1_Account
    Current User: xxUserxx (005A0000005FohB)
    Start time: 3/2/2016 2:24 PM
    Duration: 28 seconds

    How the Interview Started
    Chris Thames (005A0000005FohB) started the flow interview.
    Some of this flow's variables were set when the interview started.
    myVariable_old = 001A0000014JsaDIAS
    myVariable_current = 001A0000014JsaDIAS
    RecursiveCountVariable = 0.00

    ASSIGNMENT: myVariable_waitStartTimeAssignment
    {!myVariable_waitStartTimeVariable} Equals {!Flow.CurrentDateTime}
    Result
    {!myVariable_waitStartTimeVariable} = "3/2/2016 2:24 PM"

    DECISION: isChangedDecision2_myRule_1_Pipeline_Rep_c
    Executed this outcome: isChangedRule_2_myRule_1_Pipeline_Rep_c
    Outcome conditions: and
    1. {!myVariable_old} (001A0000014JsaDIAS) Is null false
    2. {!myVariable_old.Pipeline_Rep__c} (005A0000007DuG5IAK) Does not equal {!myVariable_current.Pipeline_Rep__c} (005A0000005FohBIAS)
    Logic: All conditions must be true (AND)

    DECISION: myDecision
    Executed this outcome: myRule_1
    Outcome conditions: and
    1. {!isChangedRule_2_myRule_1_Pipeline_Rep_c} (true) Equals true
    2. {!myVariable_current.Pipeline_Rep__c} (005A0000005FohBIAS) Is null false
    Logic: All conditions must be true (AND)

    RECORD UPDATE: myRule_1_A1
    Find all Contact records where:
    AccountId Equals {!myVariable_current.Id} (001A0000014JsaDIAS)
    Update the records’ field values.
    OwnerId = {!myVariable_current.Pipeline_Rep__c} (005A0000005FohBIAS)
    Result
    Failed to update records that meet the filter criteria.

    Error Occurred: UPDATE --- UPDATE FAILED --- ERRORS : (ALL_OR_NONE_OPERATION_ROLLED_BACK) Record rolled back because not all records were valid and the request was using AllOrNone header --- for SFDC record with ID : 003A000001VHDehIAH,