As zippykona says, most likely to be tyre pressure. Given the generally low pressures that fat tyres are run at then any change whilst low in numerical terms is actually pretty big in percentage terms. When I got my fat bike I was getting understeer and generally weird handling especially on roads – checked the pressure 7.5psi. Pumped up to 8psi and the problem went away.
Get a decent pressure gauge, one that is for low pressures, start with 1psi for every 10Kg of rider and kit weight then adjust from there in 0.5psi increments/decrements until you get the behaviour you want. If you change tyres then start again as every model is different.