I don't think it's been pumping for Pizza Day. Because Pizza Day is not going to repeat again. No one is going to buy pizza with such a big amount. Bitcoin has been trying to break its ATH for a few days, as we can see. So it isn't a hard pump, really; it probably just increased 5% from the last day, and it does cross ATH. A couple of institutional investors seems accumulating more Bitcoin; hence, it's been pumping a bit. This means demand has increased for Bitcoin.
It is possible due to the promotion of Bitcoin in this Pizza day. Yes I agree that Bitcoin had been trying to break its ATH in the past days and today, with the help of the Pizza day celebration Bitcoin break its ATH. It is not surprising because Bitcoin has been ready to take off since last week and now the hype of the celebration day strengthens that rally that resulted in breaking the ATH.
Instead of thinking about why it's been pumping, we may accumulate when it dumps again. People often miss the accumulation zone and why pumping just makes us regret it. We have to find out how many we take advantage of the pump.
I highly agree, others even short their trade to take profit and wait till the market dumps before buying again. This way they can increase their stash of Bitcoin and accumulate more than what they sold at a higher price.