This is a common observation and this cycles repeats every 3 months or so. BTC rises and stabilizes, Old Alts get cheaper. People buy favorite old alts like Lisk, EOS, Stratis. Then a new wave of people come in and get scammed by pump groups to buy the cheap ones which rise in value too. They then get dumped over and confidence falls leading to the dips. Cycle repeats.
I noticed that as bitcoinBTC grows, other coins lag behind by 15-30%, and then catch up, why they do not grow at the same time?
Bitcoin is the easiest to directly buy with Fiat by new customers. Even those who keep their fiat as USDT(Tether) on exchanges refill their BTC first whenever their are positive signals. It is clear to everyone that bitcoin is the only solid bet while the others are mostly good for speculative, short-term earnings.
So everybody tops up bitcoin raising the price of BTC. Then when it looks like the market has stabilized sufficiently, people want to catch onto the next Alt with "potential" and trade their BTC for it.
Hardly any fiat enters alts through exchanges.
Due to high price of Bitcoin overall, they can invest a lot in lower coins that also contains the potential for its price to rise up. N
This too is a good reason that why alts start pumping after BTC. Because all alts drop in BTC value on exchanges. When it stabilizes, people want to get more of their favorite alts which are now cheaper because of BTC rise.