That to me sounds like this is not actual closeness. Definitely not "super close" like the wording in your later post. Just a phase of getting closer. Getting attached. Where you start to sense the risks of getting more involved. Because the relationship is about to get deeper with actual attachment. That's okay to not want to trust so fast imo.
There are a lot of reasons why it's normal to not want to trust too fast. You do need to know the person (and yourself) well enough to know how much you can share with them, you do need to have good enough people knowledge to judge that confidently enough, and even then it will still be a risk.
Even when you know how to not put all your eggs in one basket and confidently manage more than one close, intimate relationship, and have enough interests in other things so you don't overinvest yourself in any one relationship and so on.
Risks like, some people will like to hit below the belt and attack vulnerabilities (or what they *think* are vulnerabilities), and no one is a saint anyway, and so on. Even when people try to do their best....Hurt or pain is unavoidable either way. It's just how it is, it's how life is.
I can't analyse this at all without specifics. If we always just use the phrasing "opening up", that is vague, undefined, blurs things here and will not lead to an answer or any kind of actual conclusion. What kinds of opening up do you mean, what kinds of things are being said and expressed, etc. or do you have examples?
Thing is again, there is more than one way to "open up" and show and express emotions and feelings.
Firstoff, it can be done with shallow emotions that pass fast or can be done with deep, attached emotions and feelings too.
What feelings are being expressed and what the emotional messages are do matter, along with the relationship context and other circumstances, so again specifics matter.
This is obviously still too general but then this is a complex topic that people have already written 1000 books about and analyse and process for years in therapy or sometimes in talks with friends.
Anyway, if you feel like you need to run when the relationship starts to actually become intimate, it sounds like to do with the avoidant attachment style too, maybe check that out. It's OK to be an avoidant btw, don't listen to anyone that says it's "wrong". It's just an attachment style.
I actually doubt other people are really attached to their coworkers either, often it's just like, they easily share shallower emotions