This is a spoiler so huhu. English is also not my first language so sorry If I can't explain it properly.
They just did the deed and when the morning comes, that is when he got to know that Taejun is a tiger. It was because of this eyes (it was glowing just like the tiger painting he saw). He run away at that time and was already preggy (tho he doesn't know it yet that he is). The lore is Tiger clan can make a human male preggy, like high chance, so the time that Yumin runaway, Taejun knows instinctly that he is preggy or will be in the next days. Chasing arc started there.

I am reading the novel and it was so funny . I feel pity for my Baby Yumin tho. Currently on ch 82 and waiting for the ch 83 to be free. I hope Taejun suffer for all the teasing and stress he gave Yumin while preggy, gosh ಠಗಠ