Infinite Loop when using vector.size() in for loop's condition statement

I was implementing Bucket Sort and using insertion sort for sorting each bucket. But when I ran the code it went into an infinite loop. After debugging, I found that the for loop was running infinitely.

// we have buckets for each intervals
// we add values into each bucket 
// each bucket is then sorted at the end
// then all the buckets are merged together
public class BucketSort {


@SuppressWarnings("unchecked")
private Vector<Double>[] buckets = new Vector[10];


private int sizeOfInput;

//constructor

public BucketSort(int size){
    sizeOfInput = size;
    for(int i=0; i<10;i++){
        buckets[i] = new Vector();

    }

}


public static void main(String args[]){
    Double[] arr =  {0.11,0.12,0.21,0.61,0.7,0.5,0.14,0.2,0.61,0.65,0.72,0.80,0.98,0.82,0.96,0.35,0.47,0.53};

    BucketSort s = new BucketSort(arr.length);

    Vector result = s.bucketSort(arr);
    System.out.println("The result is : ");

    for(int i= 0;i< result.size();i++){
            System.out.println(result.get(i));
    }
    //s.printBucket();
} 

public void printBucket(){
    // prints out the elements in each bucket 
    for(int i = 0; i<10;i++){
        System.out.println("The bucket "+ i + " contains these elements");
        for(int j=0; j<buckets[i].size();j++){

                System.out.println(buckets[i].get(j));
        }
    }
}


public Vector bucketSort(Double[] arr){
    // add the elements in appropriate buckets
    for(int i = 0; i<arr.length;i++){
        for(int j=0; j<10;j++){
            if(arr[i] < (double)(j+1)/10){
                buckets[j].add((double)arr[i]);
                break;
            }
        }
    }

    // print out each bucket
    printBucket();

    // call the sort function on each of the buckets
    for(int i= 0 ; i< 1; i++){

        sort(1);
        System.out.println("Sorted bucket" + i);
    }


    return merge();
}

public void sort(int number){

    //binary sort each values of vector
    System.out.println("bucket" + number + "size is " + buckets[number].size());
    int k = buckets[number].size();

    for(int i = 1; i < (buckets[number].size()); i++ ){
        System.out.println("Bucket" + number + "element" + i);

        double a = buckets[number].get(i); 
        int j= i;
        while(j > 0 && buckets[number].get(j-1) > a){
                //double b = buckets[no].get(j-1);
                //buckets[no].add(j,b);
                j--;
                System.out.println("Sorting bucket" + number);
        }

        buckets[number].add(j,a);           

    }
}

public Vector merge(){
    //merge all the bucket vectors into an array
    Vector<Double> result = new Vector();

        for(int j=0;j<10;j++){
            //buckets[j].copyInto(result);
            result.addAll(buckets[j]);
        }

    return result;
}   

}

If I replace

for(int i = 1; i < (buckets[number].size()); i++ ){

with

for(int i = 1; i < k; i++ ){

where k = buckets[number].size()

This solved the problem but I didn't understand why did the error occur in the first place. Please explain the reason for such inappropriate behavior.