Why are there so many pokemon in one spot?

Hello all, i’m new to this forum. I’ve been playing for about 2 months now, but just came across something odd. I stopped at a gas station on my way home and it had a crazy amount of pokemon there. Theres not pokestop close by, no gym, and I wasnt using incense. Anyone else ever seen this? I also noticed that all of the pokemon were higher level than usual, like the ponyta I caught there at 1203CP. Just thought it was odd. Any input? Sorry it wont let me post the pictures for some reason. But, I had 17 pokemon surrounding me at this spot.

Spawns normally increase in both number and rarity (including CP / IVs) with mobile / cellular phone use. Lots of people checking their phones st the gas station which will increase the number of spawns.
A good example is below (posting this at 14:09 14/12/17 & pic was taken 5 mins ago) - I work right near the main mall in my city & because of the increased signal, more & better spawns appear.


That makes sense! Thanks! That blew my mind yesterday.

This is called a cluster spawn. They are common at places with high volume of people such as petrol stations.