Digital Twins (DTs) are leading-edge technologies in Industry 4.0 that facilitates monitoring, real-time simulation and optimization of physical systems. However, their significant integration with Cyber-Physical Systems (CPS) and Internet of Things (IoT) makes them vulnerable to crucial cyber-attacks. One of them are Distributed Denial of Service (DDoS) attacks which causes a major risk to the integrity of data and operational continuity. This study methodically identifies and ranks the primary factors leading to DDoS attacks on Digital Twin systems. Implementing an integrated Multi-Criteria Decision-Making (MCDM) method, the Analytic Hierarchy Process (AHP) is employed to set the weights of evaluation criteria through expert opinions, whereas the Technique for Order of Preference by Similarity to Ideal Solution (TOPSIS) is applied to rank the identified causative factors. The findings indicate that the lack of distributed threat detection, vulnerable network protocols and an extensive attack surface represent the most critical attack factors. A comprehensive sensitivity analysis validates the stability of these rankings, offering a justified prioritization to support the development of targeted and effective defense strategies for securing the implementations of DTs against alarming cyber-attacks.