Motion blur only at back of moving subject - why?

I do aviation stuff - I've noticed frequently that the central and front part of the aircraft is sharp, while the back has a noticeable amount of what looks like motion blur. I don't understand why this is happening - any thoughts?

Example:
https://imgur.com/a/bYoy7pb

This is a pretty rare aircraft, a Boeing 727, which there aren't many of left flying, and somehow the tail/empenage is blurred here while the front end looks sharp. Really disappointed that the shots didn't turn out totally sharp…

I have pretty old gear, so that might be part of it - this is a Canon 5D Mark IV with an EF 100-400 f/4.5-5.6L Mark 1 (the "dust pumper” push/pull design) that's like 25 years old but still in good condition. Settings were manual, 1/1250s, f/5.0, ISO 320 (auto). Honestly it was probably a bit underexposed due to bright sky/clouds behind, I was on centerweighted with +1/3 EV, but this is a constant struggle with this kind of shot trying to get the metering correct.

Appreciate any help here!