The issue here is that buprenorphine (the active substance in suboxone) is a high-affinity partial agonist.
Basically, normal opioids like oxycodone fully active your opioid receptors, but do not bind to them for long.
Buprenorphine, on the other hand, is fairly weak at activating a receptor, but it will bind to the receptor more strongly and for much longer.
At low doses, both of these can cause typical opioid effects. However, buprenorphine soon hits a "ceiling" where a greater number of receptors being activated can no longer make up for buprenorphine's inability to effectively stimulate each individual receptor. At >4mg of buprenorphine, the bupe molecules have to compete for the receptors amongst themselves, and once you take the dosage beyond ~8mg any further increase may not significantly improve its painkilling/euphoric effects.
And since bupe is much better at binding to the receptors than oxycodone, a large dose of buprenorphine can just displace a large dose of oxycodone from the receptors, causing instant withdrawal symptoms in an opioid-tolerant patient (this is why people are not supposed to transition from heroin to a suboxone maintenance treatment before they're already in withdrawal).
