Ship goes into suborbital randomly while rendezvous

[Cat91]

As title, I've already put a station/ship in LKO, when it flies to the right spot, I launch another ship to it. While I was doing a rendezvous, the ship just suddenly set itself into suborbital flight for no reason, and after a short while it crash into terrain. So I have to use cheat to set it back to orbit to prevent it from crashing, then do the whole thing again. This makes me really frustrated . Can someone help me?

Also, I can't use map view often, because that makes the game crash, so I don't really know when does it happens...

[Starman4308]

First, are you close to the edge of the atmosphere? If your orbit closely skims the atmosphere, even small changes in velocity could send you suborbital.

Second, please read this: