Very, very, likely there is no deeper reason.
You are probably experiencing what Daniel Kahneman calls "the law of small numbers" (in his "Thinking, Fast and Slow"). We expect small samples to have the same behaviour as the underlying, large, universe they are drawn from. So we may expect to have also roughly 50:50 birth distribution on a small sample, because we know that is so on the large scale. Yet that is not the case, "strange" things this happen, more so on the small scale.
A couple of quick reasonings.
There are some 135 million births per year (2016 data, from wikipedia). IF you split those in batches of 100 (I do not know how many you have observed), that means we have about 1.35 million batches. Now note that something as (un)likely as 1 in 1.35 million (0.00007%) is quite likely (63%) to happen in at least one of those batches.
Another approach, a quick Bayesian estimate of the birth distribution updated by your observation. Using a prior knowledge of binomial distribution with 67.5 million newborns of each gender, that is a probability of 50% of having a girl on a birth. A updated best guess for the newborn girl probability, using 0.7*p girls out of p births, would be:
For $p=100, P(girl,p) \approx 50.000015%$.
Finaly, last assume in the previous 4 years you had a birth distribution of 45:55, (girls:boys). I bet you would not take any special note of it. Now if you join those more or less normal years and add them to this strange year, you get $(4*45+1*70):(4*55+1*30) \rightarrow 250:250 \rightarrow 50:50$, that is, a completely normal 5 year period.