period和loan一起做回归就出现了这种情况

. logit schedule period loan

Iteration 0:   log likelihood = -65818.578  
Iteration 1:   log likelihood = -64067.481  
Iteration 2:   log likelihood = -62353.435  
Iteration 3:   log likelihood = -60212.509  
Iteration 4:   log likelihood = -59835.765  
Iteration 5:   log likelihood = -59829.737  
Iteration 6:   log likelihood = -59829.737  (backed up)
Iteration 7:   log likelihood = -59829.737  (backed up)
Iteration 8:   log likelihood = -59829.737  (backed up)
Iteration 9:   log likelihood = -59829.737  (backed up)
Iteration 10:  log likelihood = -59829.737  (backed up)
Iteration 11:  log likelihood = -59829.737  (backed up)
Iteration 12:  log likelihood = -59829.737  (backed up)
Iteration 13:  log likelihood = -59829.737  (backed up)
Iteration 14:  log likelihood = -59829.737  (backed up)

和其他变量在一起则完全没问题，如下表。loan变量和其他变量一起也完全没问题
. logit  schedule period inscore outscore

Iteration 0:   log likelihood = -65818.578  
Iteration 1:   log likelihood = -48557.048  
Iteration 2:   log likelihood = -47195.366  
Iteration 3:   log likelihood = -47058.916  
Iteration 4:   log likelihood = -47051.595  
Iteration 5:   log likelihood = -47051.583  
Iteration 6:   log likelihood = -47051.583  

Logistic regression                               Number of obs   =      97224
                                                  LR chi2(3)      =   37533.99
                                                  Prob > chi2     =     0.0000
Log likelihood = -47051.583                       Pseudo R2       =     0.2851

------------------------------------------------------------------------------
    schedule |      Coef.   Std. Err.      z    P>|z|     [95% Conf. Interval]
-------------+----------------------------------------------------------------
      period |   -.051859   .0024789   -20.92   0.000    -.0567175   -.0470005
     inscore |   .0809126   .0005939   136.23   0.000     .0797485    .0820767
    outscore |   .0001233   4.76e-06    25.91   0.000     .0001139    .0001326
       _cons |  -1.496561   .0235419   -63.57   0.000    -1.542702    -1.45042
------------------------------------------------------------------------------
本人计量原理学的不是很好，求高人指点 这不停的Backed up是怎么了？