The increase in new anti-immigration rules of the UK and the absence of legal routes have resulted in many deaths among migrants attempting to cross the English Channel irregularly. Currently, little is known about the factors influencing irregular migration, especially in transit countries. Between 2018 and 2021 there was a significant rise in illegalized migrants trying to reach the UK by hiding in trucks heading for ferries departing from the port of Rotterdam. In 2021, there was a sudden drop in these migrant numbers, lasting until the end of 2023. Through an instrumental case study in the port of Rotterdam, this thesis seeks to explain this change in the number of illegalized migrants. The data collection will be done through a review of existing research based on the Hagen-Zanker and Mallet's approach and semi-structured interviews with relevant parties such as border police and humanitarian organizations in the Netherlands and the UK. The findings of this thesis indicate that increased border control in Rotterdam caused migrants to move to other locations. Additionally, human smugglers played a key role in determining the routes taken by migrants heading to the UK. This research highlights how conditions in transit countries impact the decisions and journeys of migrants. Furthermore, it emphasizes the limited understanding of migration factors in transit countries, underscoring the need for further research in this area.