BUG: astype() on an integer DataFrame changes the order of data
- [x] I have checked that this issue has not already been reported.

- [x] I have confirmed this bug exists on the latest version of pandas.

- [ ] (optional) I have confirmed this bug exists on the master branch of pandas.

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**Note**: Please read [this guide](https://matthewrocklin.com/blog/work/2018/02/28/minimal-bug-reports) detailing how to provide the necessary information for us to reproduce your bug.

#### Code Sample, a copy-pastable example

```python
import numpy as np
import pandas as pd
npa = np.random.RandomState(0).randint(1000, size=(20, 8))
df = pd.DataFrame(npa, columns=[f'c{i}' for i in range(8)])
print(df.iloc[:6, :3])
#     c0   c1   c2
# 0  684  559  629
# 1    9  723  277
# 2  600  396  314
# 3  600  849  677
# 4  115  976  755
# 5   99  984  177
print(df.iloc[:6, :3].astype('int32'))
#     c0   c1   c2
# 0  684  600  115
# 1  559  396  976
# 2  629  314  755
# 3    9  600   99
# 4  723  849  984
# 5  277  677  177
```

#### Problem description

The order of output result is changed unexpectedly after `astype` is called. It may caused by data continuity as `df.iloc[:6, :3].copy().astype('int32')` works as expected.

#### Expected Output

```python
print(df.iloc[:6, :3].astype('int32'))
#     c0   c1   c2
# 0  684  559  629
# 1    9  723  277
# 2  600  396  314
# 3  600  849  677
# 4  115  976  755
# 5   99  984  177
```

#### Output of ``pd.show_versions()``

<details>

INSTALLED VERSIONS
------------------
commit           : f00ed8f47020034e752baf0250483053340971b0
python           : 3.8.5.final.0
python-bits      : 64
OS               : Darwin
OS-release       : 19.6.0
Version          : Darwin Kernel Version 19.6.0: Thu May  6 00:48:39 PDT 2021; root:xnu-6153.141.33~1/RELEASE_X86_64
machine          : x86_64
processor        : i386
byteorder        : little
LC_ALL           : None
LANG             : None
LOCALE           : zh_CN.UTF-8
pandas           : 1.3.0
numpy            : 1.18.5
pytz             : 2021.1
dateutil         : 2.8.1
pip              : 20.2.2
setuptools       : 49.6.0.post20200814
Cython           : 0.29.23
pytest           : 6.2.2
hypothesis       : 6.3.0
sphinx           : 3.2.1
blosc            : None
feather          : None
xlsxwriter       : None
lxml.etree       : None
html5lib         : None
pymysql          : None
psycopg2         : None
jinja2           : 2.11.2
IPython          : 7.17.0
pandas_datareader: None
bs4              : None
bottleneck       : None
fsspec           : None
fastparquet      : 0.4.2
gcsfs            : None
matplotlib       : 3.2.2
numexpr          : 2.7.1
odfpy            : None
openpyxl         : None
pandas_gbq       : None
pyarrow          : 4.0.0
pyxlsb           : None
s3fs             : None
scipy            : 1.6.0
sqlalchemy       : 1.3.18
tables           : None
tabulate         : None
xarray           : None
xlrd             : None
xlwt             : None
numba            : 0.53.0

</details>
