There are three possibilities that could be considered "removed":
1) the post fell off the 'new' posts page. This is normal, the 'new' posts page is simply a reverse time ordered list of posts. So a sufficient number of new posts arriving quickly will push any given post off the first page of new items onto subsequent pages of the new list.
2) the post made it to the HN front page, but then disappeared from the HN front page. Posts make it onto the HN front page by receiving up votes from users, and the algorithm has some time based considerations that it uses to decide if a post qualifies to be one of the posts listed on the first section of the front page. The speed with which upvotes arrive, and how many upvotes in total, seem to have a bearing on a post being on the main page. So if this is the case, it is possible it got enough upvotes rapidly enough to make it to the main page, but did not receive enough additional upvotes once there to remain eligible to be on the front page. This is also normal, and is just how HN's front page algorithm works.
3) the third possibility is the post was flagged. In this case, a sufficient number of users pressed the "flag" link on the post to cause the software to mark it "flagged" and remove it from view. Note that you need a certain level of karma points before you gain the ability to flag a post. If the post was flagged, however, you would see that fact in the 'submissions' section under your account name. The post title will begin with "[flagged]" if this is what occurred. This is also normal, however the guidelines indicate that flagging should generally be used for spam or off-topic posts (https://news.ycombinator.com/newsguidelines.html) so if the post was flagged, you might consider comparing the post's contents with the guidelines to see if it might have been considered off-topic or spam by some.