Name changed as some information here is quite outing... Sorry, bit of an essay...
My DM's husband/stepdad has been in my life for some 20+ years now and has been very supportive in a lot of aspects (financially, encouraging regarding education, hobbies etc) but has a tendency to really fly off the handle with me or my siblings if there's even the slightest heated discussion/confrontation. This has been a constant theme over the course of growing up and into adulthood, in particular with me and my DSis. I always assumed it was usual clashing with teenage stepkids, plus he can be quite a gruff personality at times... But even now that we are all adults, in situations where we bicker (and you know, it happens, because we're family and that's inevitable), he immediately flies off the handle, shouts, swears, acts aggressively, then usually storms off and refuses to communicate with us for a long time. This is more often than not in situations/conversations where I just wouldn't deem it necessary to react so strongly.
Just as an example... A few months ago, he and DM booked a holiday and assumed my DSis would be able to house sit for them on those dates for 10 days so they wouldn't have to put their dogs in a kennel. DSis has a disabled child and their house just isn't kitted out for being able to look after him easily there, not to mention space issues, kids routines, school runs, work etc. When she tried to point out to him that it would be quite difficult for her, he started shouting that he'd "just have the dogs put down then", swore at her in front of her kids because he "couldn't believe her attitude after everything they do for her" and stormed off. He didn't speak to her for nearly a month afterwards and then just quietly pretended like nothing had happened when he tagged along for a visit to her house with my DM. This is quite typical unfortunately.
Anyway, fast forward to last weekend... I ended up having a discussion with him which became heated (I won't go into detail on what exactly it was regarding as that's not really relevant but it did have a political aspect to it and we both turned out to feel differently on the subject). Long story short but what started out as a pretty mundane current affairs conversation turned into him hurling insults at me over messenger, then blocking me and my DH (who had absolutely nothing to do with the conversation in any way!) on all forms of social media, WhatsApp, phone etc. My mum, as always, is dismissing the fact that he's taken the situation from 0-100mph totally unnecessarily...
I realise he'll probably give me the silent treatment for a good few weeks and then he'll more than likely go back to acting like nothing ever happened, I'm guessing when I give birth in a few weeks time (currently 37 weeks pregnant). But this situation happening for what feels like the millionth time over something like this has made me start to question yet again how toxic the relationship really is if this is something I'm always nervous of and what I can do, if anything, about it. Talking to him in calmer moments doesn't work because it always goes the same way with him getting defensive and the cycle repeats. I've tried several times to discuss with my DM and ask her if she thinks that his behaviour is acceptable but she always acts very dismissive and defends him, usually by arguing that me and/or my siblings deliberately rile him up and that actually he has done a lot for us (not disputing that), which somehow makes it all fine to her?
DH is frankly so very fed up of this constantly happening (can't say I blame him) and, while not actively involving himself in arguments, has told me privately that he thinks this isn't okay and doesn't understand why I don't just distance myself because nothing else seems to have worked. But (when she's not sticking up for him in these situations) I am very close to my DM and find the idea of distancing myself from him impossible without it impacting my relationship with her. WIBU to back away from them because of this? What else can I do? I'm probably really feeling it this time along with pregnancy hormones but just in general it doesn't feel healthy to always have the possibility of this happening hanging over me 
Thank you so much if you've got this far. I'm not trying to paint myself as totally blameless in the arguments by the way - it's not so much that they happen at all as it is about the way that he conducts himself when it does, IYSWIM.