There’s very little to go on.
But it sounds like she’s struggling with her mental health. When a mother has carried most of the physical, mental and emotional load for raising children, she can be absolutely exhausted (and have lost any sense of herself) by the teenage years. And these can be the hardest, as teens are difficult to ‘control’ and can be very ungrateful and nasty to parents. Throw in the perimenopause and it can push some women over the edge.
Family fall outs, with people who they’ve always got on with, can be a sign of poor mental health. When unwell, the slightest things can be heightened and raw. People seeming unempathetic or unsupportive, let alone harsh or judgemental, can be very difficult to cope with. People often withdraw because they feel so unloved, so ashamed they’re not coping or worried they’ll say something they’ll regret - or already have.
If you love your wife, suggest couples counselling on the basis you really want to listen and understand how she feels. And find solutions, even if it’s simply as supportive co-parents for now. You may be able to get it for free via the school, who should be told in any case, so they can support the kids, especially the 8yo if he’s struggling.
IMHO, she shouldn’t have rejected your 15yo. That’s unkind and is likely to be very damaging to him - and his little brother. Rebuilding a relationship with him must be a priority, even if it’s just the odd walk and coffee. Teenage years can be very hard, as you have to set clear boundaries about respect, but stop speaking to them as ‘children’ and start to see them as young adults, who have to take responsibility for themselves and their behaviour.
While this is being resolved, be there as much as you can for your lads, even if you have to reduce your working hours for a while. Help them feel together with you as a family and have lots of fun, as that helps kids cope, e.g. crazy golf followed by a walk and lunch or card games of an evening. Be around a lot, as that helps them open up, e.g. reading with the 8 yo in bed. Never seem too busy to listen, even if you’re in a real rush.
Be as honest as possible with them. If you don’t really know why she’s left, tell them you don’t really know, but you believe she loves them very much and wants the best for them, but is struggling at the moment and needs some time to get stronger. And you all have to do your best to understand and help because that’s what families have to do. Never criticise her to them. But encourage them to let you know if her behaviour upsets them.
Good luck. I hope this makes you stronger as a family 💐