Belgian Malinois – understanding triggers?

For context, the dog I'm about to talk about is not my own – but is my parent's dog (Male Belgian Malinois approx 5 years old). He is a sweet boy, but does have some weird behavioural problems that I'm wondering if anyone has seen before in the breed or has any idea what the root triggers might be. I'm someone that does work in the veterinary industry, and I had originally advised my parents to stick with a breed that is more suited to home living. But alas, my Dad fell in love with him and decided to go with it. They adopted him through a rescue program, but we aren't too sure about his previous history other than he was getting trained for bomb sniffing at one point. My Dad pours his heart and soul into this dog – and has done his best to keep him active (3-4 walks or outdoor play sessions, sometimes more daily + agility training).

While they are putting in that effort for him, this dog has seemingly random moments of high-arousal that can be intimidating. What I mean by this, is that if I go too quickly up the stairs the dog immediately enters a high state of arousal and will charge, bark a lot and jump up at me. While this hasn't resulted in any nipping, I feel like if it were someone else who was afraid of him that it could potential lead to this. This dog does have a bite history as well (Did not take to boarding well at all), so naturally when he acts that way it is a little scary. This same behaviour/reaction can also come out if myself and my parents are engaged in conversation in the hallway – but the dog will come directly out at me (I'm assuming since I only stay at their place every once in a while so I am otherwise a stranger). It's definitely an intimidating thing to be on the receiving end of, but I'm not out here to blame the dog. He's otherwise social and loves a good head scratch from time to time. It just seems like he has these triggers socially or physically that put him in a state of high arousal/anxiety? I'm just wondering if anyone has any thoughts or advice on the matter.

I have advised my parents on multiple occasions to get in touch with a behaviouralist, but they have yet to do so. Please be kind, I am not the owner of this dog – but am just looking to see if I can come up with any other suggestions or ideas to help them out.