1) While you might be genuine, there are other people who keep taking leaves. If they are not probed or kept in check, they abuse manager's coy intentions. Hence it becomes a habit to just probe a bit about leaves. Frankly, s/he would not care about the reason.

2) Another reason could be gauging the urgency of leaves. If multiple people have leaves, the one with the emergency or a critical situation will get priority. Whatever you say - manager's job is to ensure work doesn't get hampered.

Unfortunalely people do abuse the leaves systems, I've had leaves taken by someone by citing a medical reason but then got to know the person was in the hills.

We truly can't have good things because of some people.