There are several reasons an official from one side of the field throws a DPI on the other side. Some are decent reasons, some are not.
If he's truly just out there officiating the whole field, he'll quickly get a rep for doing this and no one will want to work with him. It's called "fishing in someone else's pond" and it's a no-no. First, he shouldn't be looking there. Second, it's hard to sell a call that's across the field. (Although fans 60 yards from plays seem to think they have a good view.)
In my crew, if you see something in someone else's area and it's a 5 or 10 yarder, let it go. If it's a 15 yarder, a major foul, it's important to get that. Even though DPI is a 15 yarder, I wouldn't throw that from across the field but maybe the official in the OP felt like he had to.
Or maybe the guy over there was very new and nervous as can be, and the older guy is forced to call most of the field. I've had it happen. Early this year I was working a 4 man crew on a JV game, (might have been middle school, can't recall) and a first year guy was shaking in his shoes. There's a long pass (25-30 yards) to his side and I peeked down there, even though normally I don't turn to watch the ball in flight. The receiver was mauled. Gross DPI, but no flag. We all saw it and we were all waiting for New Guy to flag it but he didn't. I'm not flagging something 45 yards away and the others felt the same way, but someone probably should have done something.