I know these letters come as a surprise. You must wonder about me.

The second segment of the line which is from a letter, seems to be expressing a probable confusion on someone after receiving
such an unexpected letter. If so, shouldn't it be like 'you must be wondering about me' instead for avoiding a obligation mood.

Yes, if I understand you correctly. 'You must wonder about me' here suggests that the recipient questions the writer's mentality. 'You must be wondering about me' suggests that the recipient may be thinking about the writer's life and activities. Better yet would be 'you must have been wondering about me', because the writer is now reestablishing communication.

The difference in probable meaning is, I think, idiomatic.
Yes, it has been very helpful. Thanks Mister Micawber.