Summary

The code in the question theoretically should not run, as the private key is in hex format and the call to new BigInteger(senderPrivKey) to parse the hex string without the 16 radix specified would result in a thrown exception.

When the code is adjusted to use new BigInteger(senderPrivKey, 16) specifying the base 16 radix, the public key is generated as expected.

The Details

Here is a working example. Save it as TestKey.java, compile and run it.

import java.math.BigInteger;

import org.ethereum.crypto.ECKey;
import org.spongycastle.util.encoders.Hex;

public class TestKey {

    public static void main(String[] args) {
        String senderPrivKey = "c85ef7d79691fe79573b1a7064c19c1a9819ebdbd1faaab1a8ec92344438aaf4";

        BigInteger pk = new BigInteger(senderPrivKey, 16);
        System.out.println("Private key: " + pk.toString(16));

        ECKey key = ECKey.fromPrivate(pk);
        System.out.println("Public key: " + Hex.toHexString(key.getAddress()));
    }
}

And the output when running it:

Private key: c85ef7d79691fe79573b1a7064c19c1a9819ebdbd1faaab1a8ec92344438aaf4
Public key: cd2a3d9f938e13cd947ec05abc7fe734df8dd826

I have used the following maven dependency:

    <dependency>
        <groupId>org.ethereum</groupId>
        <artifactId>ethereumj-core</artifactId>
        <version>1.1.0-RELEASE</version>
        <!--  <type>zip</type>  -->
    </dependency>

I don't know how your code ran, because when I change the following line in the source code above to match your code, the BigInteger(...) constructor is expecting to parse a base 10 number rather than a base 16 number :

        BigInteger pk = new BigInteger(senderPrivKey);

And I get the following exception:

Exception in thread "main" java.lang.NumberFormatException: For input string: "c"
    at java.lang.NumberFormatException.forInputString(NumberFormatException.java:65)
    at java.lang.Integer.parseInt(Integer.java:580)
    at java.math.BigInteger.<init>(BigInteger.java:461)
    at java.math.BigInteger.<init>(BigInteger.java:597)
    at TestKey.main(TestKey.java:14)

To generate the Credentials if you have a plain public key and private key, you need to convert them in to the hex representation and then pass them in to the constructor for Credentials

String hexPrivateKey = String.format("%040x", new BigInteger(1, privateKey.getBytes()));
String hexPublicKey = String.format("%040x", new BigInteger(1, publicKey.getBytes()));  
Credentials creds = Credentials.create(hexPrivateKey, hexPublicKey);

Where publicKey and privateKey are Strings containing your plain pub and priv keys