Why would a guy come back after ghosting you?

Read the message for specifics before you read it for warmth. If he names something you said, asks about a plan you made, or apologises for the gap in his own words, he's putting in effort to re-enter your life. If the message could have been sent to anyone, with no reference to you or to the conversation you were having, he's testing the water and expecting you to do the rest. Neither reading is automatically a red flag, but the difference is worth noticing. The silence was his choice, and the comeback is his second choice. How he handles the second one tells you more than the excuse he offers for the first.

Also notice what he's asking you for. A comeback that wants reassurance, that asks you to confirm you're not upset or to tell him it's fine, is quietly moving the emotional work onto you. A comeback that simply makes contact and leaves room for you to decide is different. You're looking for someone who can take a bit of responsibility without needing you to absolve him.

Your reply should acknowledge that there was a gap, in a line, and then move on. Naming the silence plainly and without drama tells him you noticed, that you're not pretending it didn't happen, and that you're not going to spend three paragraphs on it either. Two kinds of reply tend to work. One is the light acknowledgement, where you mention the gap briefly and then answer his question or add something of your own. The other is the open door with a condition, where you say you're happy to hear from him but would like a bit more than a hey before you pick things back up. Both ask for nothing you can't reasonably ask for.

What both should leave out is any attempt to diagnose him. Don't speculate about why he went quiet, don't list the ways it affected you, and don't frame your reply as a test. Keep the message about what you're willing to do next, not about what he did. A draft that is mostly about his behaviour will keep the conversation about his behaviour for the next several messages.

Wait a reasonable amount of time before you answer. A few hours is usually fine, and within a day is normal. Waiting several days to make a point tends to read as a game, and answering in seconds reads as eager, which tells him the silence cost you nothing. Choose a time when you're relaxed and not already annoyed, because a reply sent while you're still stewing will carry that tone whether you mean it to or not. If it's late and he's messaging after a long gap, it's fine to reply in the morning. The comeback doesn't have an expiry date.

If you reply and he goes quiet again, you don't need to chase. One follow-up is reasonable if the first reply was warm and he didn't answer. After that, the most useful thing you can do is let the silence be his. If he returns a second time with the same vagueness, you have your answer about how he handles this sort of thing, and you can decide whether to keep the door open or close it without a dramatic message. Stopping is a valid outcome, and it doesn't require you to explain yourself.