Parllel Execution Failing randomly

I have created a maven demo framework (BDD+POM), If I am executing scripts on single thread it working fine without any issue. However if I increase the thread count more then 1, scripts are failing randomly on random line for every execution.

I am attaching my sample project and below are the steps/requirment to run the test cases:

1. TestNG plugin for eclipse

2. Select browser - update the value of variable "browserName" in file BDD_DemoProject\src\main\java\com\selenium\demo\utility\Constants.java to Mozilla or Chrome

2. Location of testng.xml file - BDD_DemoProject/resources/testng.xml

3. Update the value attribute "data-provider-thread-count" in testng.xml to change the number of threads

4. To run the test case - Run testng.xml as TestNG Suite

Please help..!!