Why do some men and women experience a sudden deterioration in their relationship after becoming intimate?

Recently, many friends have said that Cat Brother's questions have been a bit "heavy."
So today, let's keep it light.
A true story (all characters are pseudonyms).
Xiao Zhang and Xiao Qian met at a friend's wedding.
Xiao Zhang was part of the groomsmen, Xiao Qian was part of the bridesmaids.
After the wedding, everyone went out for drinks and played games in the bridal chamber, creating a lively atmosphere.
That night, the two of them hit it off very well.
Under the influence of alcohol and the festive mood, they happened to have a chance to be alone.
The two became intimate.
The next day, Xiao Qian had to go on a business trip out of town.
According to Xiao Zhang, they still felt quite tender when saying goodbye.
Xiao Qian has a sweet appearance and loves to smile; Xiao Zhang liked her quite a bit at the time.
They continued to keep in touch afterward.
Xiao Zhang is single, and Xiao Qian also said she didn't have a boyfriend.
Initially, their chats were pretty good.
Xiao Zhang asked, "What are you doing, Qian Qian?"
Xiao Qian would send photos:
"Having hotpot with colleagues."
Xiao Zhang asked what kind of work she does.
Xiao Qian said she is a graphic artist at a gaming company.
Xiao Zhang also said he runs a small business, owns a shop.
Once, Xiao Zhang even asked:
"What do you think of me?"
Xiao Qian replied:
"You're pretty good, but I don't know you very well yet. We can get to know each other slowly."
Everything was pretty normal up to this point.
But then, things started to change gradually.
Xiao Zhang still often initiated conversations.
But Xiao Qian's replies began to slow down.
Sometimes it took hours.
Sometimes she wouldn't reply until the next day.
The reasons were mostly:
"Work has been busy lately, sorry about that."
Until one day, Xiao Zhang couldn't help but test the waters and asked:
"Should we try being official?"
Xiao Qian only replied with one sentence:
"I don't think we're quite compatible."
After that, she basically stopped replying.
Xiao Zhang couldn't understand one thing:
That night, the atmosphere was great.
There was no awkwardness when saying goodbye.
Their initial chats were quite normal.
Why did it suddenly turn like this after a few days?
What do you think is the most likely reason?
(Cat Brother also encountered almost the exact same situation years ago, so he's quite curious about everyone's opinions.)