The combined market share of the two companies threatens Samsung's position as the world's biggest NAND memory producer. Western Digital and Kioxia (Toshiba's memory division) have already been collaborating for 20 years, so the merger does not come as a huge surprise.
https://www.notebookcheck.net/Western-Digital-in-talks-with-Kioxia-for-a-20-billion-merger.556901.0.html