There are a number of reasons for the spikes on the edges of kiln formed glass. Without a picture it is difficult to help because so many assumptions have to be made.
If the edges are spiked the classic reasons for that are: a make up of less than 6mm, firing too hot, firing on fibre paper or batt wash rather than thinfire, glass touching something during the firing.
If the edges are rough -as opposed to spiked – the classic reasons are: ground edges without proper cleaning, thinfire curling in and leaving a deposit.
There are, of course, other reasons, but these seem to be the most common.
I have assumed you are using fusing compatible glass. If not, then there are a whole range of other factors that can give difficulty.
Othere have more experience than I do with microwave kilns, but I think you probably are too hot in your firing.