What is the cause of when power starts declining and heart rate goes down during an interval? Underfueling, (lactic) acid build up, something else?
Fatigue
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So lactic acid build up?
@ArHu74 depends on the interval. If it’s a 90 minute Tempo interval (colloquially known as Vaughters interval 
) I’d say that’s just normal fatigue exacerbated by poor hydration.
If it’s a VO2max interval I’d say check your HR strap & if that seems OK see a doctor.
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Also, as you get further into an interval you’ll likely start getting into bigger motor units which are less aerobically well-trained. Keep going and they get better trained, eventually. 
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No, most probably you just don’t have the aerobic endurance to maintain the workload.
To remedy that, do slightly less hard work next time.
Other factors can be sleep, nutrition, health.
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I agree with this. Fatigue.
Happens to me in VO2 Max Intervals where I come out too hot, or if I’m not fully recovered going in. By the last interval I’m not able to maintain the same power nor can I get the same heart rate response.
Conversely, come in well rested, and/or pace it better, and I’m able to build power (and heart rate) through to the last interval.
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You can simply say power declines, I got a little confused at first.
Perhaps something else to consider - what is the cause of increasing power (or duration) over a set of intervals? Being serious.
Sometimes power starts declining but heart rates starts increasing, I figured this was caused by fatigue/lactic acid build up. Ending intervals like that feels much more tiring but also satisfying